feat(docs): add a Diátaxis documentation section to the public site

A new public /docs area covering the whole platform, organized by Diátaxis:
tutorials (learning), how-to guides (tasks), reference (facts), and explanation
(the why). 29 pages, each grounded in the actual routes, actions, schema, and
UI — verified against the code and corrected for accuracy (e.g. admin-only vs
admin+board gating, where the permissions matrix actually lives, how Stripe
reconciliation matches).

Implementation:
- lib/docs/nav.ts is the single source of truth (categories + pages), driving
  the sidebar, static params, and prev/next.
- Content authored as content/docs/<category>/<slug>.md, read at build time
  (the routes are statically generated), rendered by <DocsMarkdown> — reuses the
  existing react-markdown/GFM stack, no new dependencies.
- (docs) route group with its own theme (docs.css) matching the public site's
  dark / monospace / lime aesthetic; sidebar with active-state, breadcrumb,
  prev/next, and a Diátaxis home. Public — signed-in users can read it too.
- "Docs" link added to the landing nav.

typecheck 0 · lint 0 errors · build ok (all /docs pages prerendered)

lib/docs/nav.ts

@@ -0,0 +1,108 @@
+// Docs information architecture, organized by the Diátaxis framework:
+//   tutorials    — learning-oriented, start-to-finish lessons
+//   how-to       — task-oriented, goal-focused recipes
+//   reference    — information-oriented, dry technical facts
+//   explanation  — understanding-oriented, background & rationale
+//
+// This registry is the single source of truth for the docs nav, the
+// static-param generation, and the content pipeline. Each page's body lives at
+// content/docs/<category>/<slug>.md and is rendered by <DocsMarkdown>.
+
+export type DocCategoryId = 'tutorials' | 'how-to' | 'reference' | 'explanation'
+
+export type DocPage = {
+  slug: string
+  title: string
+  summary: string
+}
+
+export type DocCategory = {
+  id: DocCategoryId
+  label: string
+  tagline: string
+  /** Diátaxis one-liner shown on the docs home. */
+  diataxis: string
+  pages: DocPage[]
+}
+
+export const DOC_CATEGORIES: DocCategory[] = [
+  {
+    id: 'tutorials',
+    label: 'Tutorials',
+    tagline: 'Start here',
+    diataxis: 'Learning-oriented lessons that take you from nothing to a working space, step by step.',
+    pages: [
+      { slug: 'getting-started', title: 'Set up your space', summary: 'Create or join a space, land as an admin, and get your bearings.' },
+      { slug: 'onboard-members', title: 'Onboard your first members', summary: 'Invite people, shape the onboarding flow, and get everyone current.' },
+      { slug: 'first-class', title: 'Run your first class', summary: 'Create a class, schedule a session, take signups, mark attendance, award a cert.' },
+      { slug: 'dues-end-to-end', title: 'Track dues, end to end', summary: 'Add a pay-here link, import a payment, and reconcile it to a member.' },
+    ],
+  },
+  {
+    id: 'how-to',
+    label: 'How-to guides',
+    tagline: 'Get a task done',
+    diataxis: 'Task-oriented recipes for a specific goal, assuming you already know the basics.',
+    pages: [
+      { slug: 'import-members', title: 'Import members from a spreadsheet', summary: 'Bulk-add members by pasting rows, with tiers and statuses.' },
+      { slug: 'invite-links', title: 'Create role-granting invite links', summary: 'Generate invite codes with usage caps, expiry, and a granted role.' },
+      { slug: 'configure-onboarding', title: 'Configure the onboarding flow', summary: 'Add, reorder, require, and disable onboarding steps.' },
+      { slug: 'connect-payments', title: 'Connect payments & reconcile dues', summary: 'Add a platform, import transactions, and match them to members.' },
+      { slug: 'build-a-form', title: 'Build a form or signable waiver', summary: 'Compose fields, set visibility, and capture immutable submissions.' },
+      { slug: 'equipment-reservations', title: 'Set up equipment & reservations', summary: 'Register tools, take reservations, and gate a tool behind a certification.' },
+      { slug: 'connect-a-door', title: 'Connect a door controller', summary: 'Wire a native or generic controller, assign cards, and read the access log.' },
+      { slug: 'run-a-proposal', title: 'Run a proposal to a vote', summary: 'Open a proposal, set quorum and threshold, and record the outcome.' },
+      { slug: 'handle-incidents', title: 'File & manage an incident', summary: 'Report an incident (anonymously if needed) and move it to a decision.' },
+      { slug: 'publish-a-policy', title: 'Publish & version a policy', summary: 'Write a policy, publish it, and supersede it with a new version.' },
+      { slug: 'customize-space', title: 'Customize roles, tiers & areas', summary: 'Rename roles, tune the permissions matrix, define tiers and areas.' },
+    ],
+  },
+  {
+    id: 'reference',
+    label: 'Reference',
+    tagline: 'Look it up',
+    diataxis: 'Information-oriented, exhaustive descriptions of the machinery. Dry and accurate.',
+    pages: [
+      { slug: 'modules', title: 'Modules overview', summary: 'Every module in one place, with what it does and where it lives.' },
+      { slug: 'roles-and-permissions', title: 'Roles & permissions', summary: 'Built-in roles, custom roles, and the per-space permission matrix.' },
+      { slug: 'members', title: 'Members: statuses, tiers & fields', summary: 'Every member status, tier, and profile field, and what each controls.' },
+      { slug: 'governance', title: 'Governance reference', summary: 'Proposal types, thresholds, quorum, incident and policy lifecycles.' },
+      { slug: 'payments-and-dues', title: 'Payments & dues reference', summary: 'Platforms, link statuses, dues advancement, and pay-here links.' },
+      { slug: 'access-control', title: 'Access control & doors reference', summary: 'Card slots, controller types, inbound ingest, and the access log.' },
+      { slug: 'forms', title: 'Forms & waivers reference', summary: 'Field types, visibility, required-form gating, and submission snapshots.' },
+      { slug: 'classes-and-equipment', title: 'Classes, certifications & equipment', summary: 'Sessions, waitlists, attendance, cert awards, and reservation rules.' },
+      { slug: 'integrations', title: 'Integrations, cron & webhooks', summary: 'Notification dispatch, door inbound webhooks, and the cron endpoints.' },
+    ],
+  },
+  {
+    id: 'explanation',
+    label: 'Explanation',
+    tagline: 'Understand why',
+    diataxis: 'Understanding-oriented discussion of how things fit together and why they are built this way.',
+    pages: [
+      { slug: 'architecture', title: 'Architecture overview', summary: 'Next.js App Router, Supabase, server actions, and the request path.' },
+      { slug: 'permissions-model', title: 'The multi-space & permissions model', summary: 'How space-scoping, roles, and effective permissions actually work.' },
+      { slug: 'governance-model', title: 'How governance works', summary: 'Why quorum, thresholds, recusal, and anonymity are modeled the way they are.' },
+      { slug: 'security-model', title: 'Security & privacy model', summary: 'RLS, the encrypted secrets vault, authz gates, and what the server derives.' },
+      { slug: 'data-and-reconciliation', title: 'Dues & reconciliation, explained', summary: 'The philosophy behind advance-only dues and hand/auto matching.' },
+    ],
+  },
+]
+
+export const DOC_CATEGORY_IDS = DOC_CATEGORIES.map(c => c.id)
+
+export function findCategory(id: string): DocCategory | undefined {
+  return DOC_CATEGORIES.find(c => c.id === id)
+}
+
+export function findPage(categoryId: string, slug: string): { category: DocCategory; page: DocPage } | undefined {
+  const category = findCategory(categoryId)
+  const page = category?.pages.find(p => p.slug === slug)
+  if (!category || !page) return undefined
+  return { category, page }
+}
+
+/** Flat, in-order list for prev/next navigation. */
+export function flatPages(): { categoryId: DocCategoryId; slug: string; title: string }[] {
+  return DOC_CATEGORIES.flatMap(c => c.pages.map(p => ({ categoryId: c.id, slug: p.slug, title: p.title })))
+}