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)

app/(docs)/layout.tsx

@@ -0,0 +1,54 @@
+import type { Metadata } from 'next'
+import Link from 'next/link'
+import { IBM_Plex_Mono, Libre_Baskerville } from 'next/font/google'
+import { BrandMark } from '@/components/brand-mark'
+import './docs.css'
+
+const ibmPlexMono = IBM_Plex_Mono({
+  subsets: ['latin'],
+  weight: ['400', '500'],
+  variable: '--font-ibm-plex-mono',
+  display: 'swap',
+})
+
+const libreBaskerville = Libre_Baskerville({
+  subsets: ['latin'],
+  weight: ['400', '700'],
+  style: ['normal', 'italic'],
+  variable: '--font-libre-baskerville',
+  display: 'swap',
+})
+
+export const metadata: Metadata = {
+  title: {
+    default: 'Docs — hackerspace.sh',
+    template: '%s — hackerspace.sh docs',
+  },
+  description:
+    'Documentation for hackerspace.sh: tutorials, how-to guides, reference, and explanation for running your space.',
+}
+
+export default function DocsLayout({ children }: { children: React.ReactNode }) {
+  return (
+    <div className={`docs-root ${ibmPlexMono.variable} ${libreBaskerville.variable}`}>
+      <div className="docs-grid-bg" aria-hidden="true" />
+      <header className="docs-header">
+        <div className="docs-header-inner">
+          <Link href="/docs" className="docs-wordmark" aria-label="hackerspace.sh docs">
+            <BrandMark className="w-5 h-5" style={{ color: 'var(--doc-accent)' }} />
+            <span>
+              hackerspace<span className="doc-accent">.sh</span>{' '}
+              <span style={{ color: 'var(--doc-text-muted)' }}>/ docs</span>
+            </span>
+          </Link>
+          <nav className="docs-header-links">
+            <Link href="/">Home</Link>
+            <Link href="/resources" className="hidden sm:inline">Resources</Link>
+            <Link href="/dashboard" className="docs-header-cta">Open app →</Link>
+          </nav>
+        </div>
+      </header>
+      {children}
+    </div>
+  )
+}