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/load.ts

@@ -0,0 +1,17 @@
+import { promises as fs } from 'node:fs'
+import path from 'node:path'
+
+// Doc bodies are authored as markdown under content/docs/<category>/<slug>.md
+// and read at build time (the /docs routes are statically generated, so the
+// files are only needed during `next build`, never at runtime).
+const CONTENT_ROOT = path.join(process.cwd(), 'content', 'docs')
+
+export async function loadDocContent(categoryId: string, slug: string): Promise<string | null> {
+  const file = path.join(CONTENT_ROOT, categoryId, `${slug}.md`)
+  try {
+    const raw = await fs.readFile(file, 'utf8')
+    return raw.trim().length > 0 ? raw : null
+  } catch {
+    return null
+  }
+}