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/run-a-proposal.md

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+Proposals give your space an auditable, async way to decide things: a member drafts a question, opens it for voting, members record a position, and the app computes quorum and pass/fail for you. This recipe walks the full lifecycle from draft to decided.
+
+## Before you start
+
+- Any current member can create a proposal and open it for voting. A member whose status is still `unverified` (pending approval) cannot create, open, or vote.
+- Only an `admin` or `board` member can mark an open proposal **decided**. A proposer can **withdraw** their own proposal only while it is still a **draft**; an `admin` or `board` member can withdraw a draft **or** an open proposal. The Withdraw button also appears to the proposer on an open proposal, but clicking it there is a silent no-op — row-level security blocks the change and no error is shown.
+- Quorum and the voting window are not entered per proposal. They are computed from your space's governance defaults at the moment voting opens. You choose only the pass **threshold**.
+
+## Create the proposal
+
+1. Go to [/proposals](/proposals) and click **New proposal** (this opens [/proposals/new](/proposals/new)).
+2. Fill in **Title** and **Body**. The body accepts Markdown and is rendered on the detail page.
+3. Pick a **Type**. This is a label describing the kind of decision:
+
+   | Type | Use for |
+   |------|---------|
+   | `advisory_poll` | Non-binding sense of the room (the default) |
+   | `board_action` | A specific action for the board to take |
+   | `membership_vote` | A decision put to the membership |
+   | `bylaw_change` | Amending a policy or bylaw |
+   | `budget` | Spending or budget approval |
+   | `recall` | A vote of confidence / recall |
+
+4. Pick a **Threshold to pass**. This sets how the yes/no split is judged when the proposal is decided:
+
+   | Threshold | Passes when |
+   |-----------|-------------|
+   | `simple_majority` | Yes votes exceed no votes |
+   | `two_thirds` | Yes is at least two-thirds of yes+no |
+   | `three_fourths` | Yes is at least three-fourths of yes+no |
+   | `unanimous` | No "no" votes and at least one "yes" |
+
+5. To go straight to voting, tick **Open voting immediately**. Its label notes that quorum and the voting window come from space defaults. Leave it unchecked to save a draft you can open later.
+6. Click **Save proposal**. You land on the proposal's detail page.
+
+## Open voting
+
+If you saved a draft, open it when you are ready. On the proposal page under **Manage**, click **Open for voting**. At that moment the app:
+
+- counts your active members and computes `quorum_required` from the space's default quorum percent (with a floor), and
+- sets the voting window using the default window length (shipped default is 216 hours, or 9 days) unless you supplied a close time.
+
+Once open, the proposal moves to the **Open for voting** section on [/proposals](/proposals) and shows its close time.
+
+## Let members vote
+
+While voting is open, every member sees a **Cast your vote** panel with four positions: `yes`, `no`, `abstain`, `recused`. Choosing `recused` requires a recusal reason. A public **Comment** is optional. Members can change their vote until the window closes; votes are never deleted. All votes and voter names are visible to the space for transparency.
+
+The **Tally** section updates live: yes / no / abstain / recused counts, plus a quorum bar showing votes counted against `quorum_required` and whether quorum is **met**. Quorum counts yes, no, and abstain; recused votes do not count toward quorum.
+
+## Record the outcome
+
+When the discussion has run its course, an `admin` or `board` member clicks **Mark decided** under **Manage**. This freezes the tally, stamps the decision time, and sets the outcome. The proposal page then shows **PASSED** or **DID NOT PASS**, and it moves to the **Archive** section on [/proposals](/proposals).
+
+A proposal passes only if quorum was met **and** the threshold you chose is satisfied on the yes/no split (abstentions do not count toward the threshold). If quorum was never met, the result is **DID NOT PASS** regardless of the vote split.
+
+## Related
+
+- [Governance model](/docs/explanation/governance-model) — why proposals, incidents, and policies are built together.
+- [Governance reference](/docs/reference/governance) — every enum, field, and status.