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/(landing)/page.tsx

@@ -44,6 +44,7 @@ export default async function LandingPage() {
             <span>hackerspace<span style={{ color: 'var(--ln-accent)' }}>.sh</span></span>
           </Link>
           <div className="landing-nav-links">
+            <Link href="/docs" className="hidden sm:inline">Docs</Link>
             <Link href="/resources" className="hidden sm:inline">Resources</Link>
             <a
               href="https://github.com/virgilvox/hackerspace-management"