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)

content/docs/how-to/invite-links.md

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+Invite codes let new people join your space through a shareable code or one-click link. Each code can grant a specific role, cap how many times it is used, and expire on a date you choose — so you can hand out a permanent public code and a single-use admin link from the same screen.
+
+## Before you start
+
+- You need the **admin** role. In the UI, creating, editing, disabling, and deleting invites is admin-only. A **board** member can open the **Invite codes** panel — it is visible to admin and board — but sees only the read-only **Copy code** and **Copy link** buttons; the **+ New invite**, **Disable/Enable**, and **Delete** controls are hidden. (The underlying `createInvite` server action also permits board, but the panel does not expose it.)
+- As the admin, you can grant any role with an invite, including **admin**.
+- Invite codes live in the **Invite codes** panel on the [`/customize`](/customize) screen.
+
+## Create an invite code
+
+1. Go to [`/customize`](/customize) and find the **Invite codes** panel.
+2. Click **+ New invite**.
+3. Fill in the fields (all optional except the granted role, which defaults to **member**):
+
+| Field | What it does |
+| --- | --- |
+| Code | The code people redeem. Leave blank to auto-generate an 8-character code. Typed codes are uppercased and limited to `A–Z`, `0–9`, and `-`, 4–32 characters, and must be unique across all spaces. |
+| Label | A private note to remember what the code is for (for example, "Open house 2026"). |
+| Expiry | A date and time after which the code stops working. Leave blank for no expiry. |
+| Max uses | How many times the code may be redeemed. Leave blank for unlimited. Must be a positive number. |
+| Grants | The role a member receives when they join with this code. The dropdown only lists roles you are allowed to assign. |
+| Single use | Sets max uses to 1 and auto-disables the code after one join. |
+
+4. Click **Create invite**. The new code appears at the top of the list with a `grants <role>` badge.
+
+## Share the code or link
+
+Each row has two buttons:
+
+- **Copy code** — copies the raw code (for example, `K7P2R9QT`) to paste into a message.
+- **Copy link** — copies a one-click join link in the form:
+
+```
+https://your-space.example/join/<space-slug>?code=<CODE>
+```
+
+The link lands new people on a space-scoped join page that carries the code into account creation. Anyone who redeems it becomes a member at the role the invite grants.
+
+## Revoke or remove a code
+
+These controls are admin-only.
+
+- **Disable** turns the code off without deleting it. Redemptions stop immediately; you can **Enable** it again later. This is the quickest way to revoke a leaked or expired code.
+- **Delete** removes the code permanently after a confirmation prompt.
+
+A code also stops working on its own once it is disabled, past its expiry, or at its use cap. The list flags these with `disabled`, `expired`, and `at cap` markers, and each row shows its usage count (for example, `uses 3 / 10`).
+
+## Notes and edge cases
+
+- Disabling and deleting affect only future joins. Members who already joined keep their role; change it from [`/members`](/members).
+- If your space requires approval, new joiners still land as unverified pending an admin's review. See [`/docs/reference/roles`](/docs/reference/roles) for what each granted role can do.