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)

components/docs/docs-sidebar.tsx

@@ -0,0 +1,33 @@
+'use client'
+
+import Link from 'next/link'
+import { usePathname } from 'next/navigation'
+import { DOC_CATEGORIES } from '@/lib/docs/nav'
+
+export function DocsSidebar() {
+  const pathname = usePathname()
+
+  return (
+    <aside className="docs-sidebar" aria-label="Docs navigation">
+      {DOC_CATEGORIES.map(cat => (
+        <div key={cat.id} className="docs-sidebar-group">
+          <p className="docs-sidebar-group-label">{cat.label}</p>
+          {cat.pages.map(page => {
+            const href = `/docs/${cat.id}/${page.slug}`
+            const isActive = pathname === href
+            return (
+              <Link
+                key={page.slug}
+                href={href}
+                className={`docs-sidebar-link${isActive ? ' is-active' : ''}`}
+                aria-current={isActive ? 'page' : undefined}
+              >
+                {page.title}
+              </Link>
+            )
+          })}
+        </div>
+      ))}
+    </aside>
+  )
+}