Add user guide, self-hosting, API, and contributing docs

docs/user-guide.md — magic-link sign-in, note basics, wikilink
syntax, keyboard shortcuts, offline behaviour, and privacy
notes (sessionStorage for tokens, tenant-scoped localStorage).

docs/self-hosting.md — system requirements, Docker Compose
quick-start, the full LIBRENOTES_* env-var matrix (which are
required, which conditional), reverse proxy snippets for Caddy
and nginx, volume layout, the in-binary healthcheck, and
update/rollback procedure.

docs/api.md — every public endpoint: auth (login/verify),
notes CRUD, /api/whoami, /healthz. Status codes per endpoint,
the optimistic-locking ?base=<unix> contract for PUT/DELETE,
note-ID regex, and the rate-limit policy.

CONTRIBUTING.md — dev setup (Nix flake .#dev, plain Go, Docker),
package layout overview, coding standards (one-way dep flow,
tenant FS gateway requirement), branch naming, commit format,
and the PR process. Also points security reports at
security@librete.ch rather than public issues.

Closes #29.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/user-guide.md

@@ -0,0 +1,83 @@
+# User guide
+
+Welcome to librenotes — an open-source, multi-tenant notes app
+with bi-directional links and Markdown.
+
+## Getting started
+
+1. Visit [https://librenot.es](https://librenot.es).
+2. Click **Sign in with email** and enter your address.
+3. Check your inbox; the magic link is valid for 15 minutes and
+   can be used once.
+4. Clicking the link signs you in and drops you into the app.
+
+There is no password to remember. Each link is a one-time
+credential; the session it creates lasts 24 hours.
+
+## Notes are Markdown files
+
+Each note is a single Markdown (`.md`) file scoped to your
+tenant directory on the server. The first H1 line is the title;
+the rest is content. You can edit notes in the web UI today; in
+a self-hosted setup you can also drop files directly into the
+data directory and they will appear in your notebook on next
+sync.
+
+```markdown
+# Project ideas
+
+A few directions worth chasing this quarter.
+
+- Try [[graph-view]] for the dashboard
+- Read [[zettelkasten-overview]]
+```
+
+### Bi-directional links (wikilinks)
+
+Use `[[note-id]]` to link to another note. The target's backlinks
+list will include the source automatically. If the target does
+not exist yet, the link will create it on click. Note IDs are
+short slugs: lowercase letters, digits, hyphens, underscores. A
+note titled "Project Ideas" might have the ID `project-ideas`.
+
+### Markdown syntax
+
+Standard CommonMark is supported: headings, lists, links, code
+blocks, fenced code with language hints, blockquotes, tables,
+images. Inline HTML is sanitised on render.
+
+## Keyboard shortcuts
+
+| Action | Shortcut |
+| ------ | -------- |
+| New note | `Ctrl+N` / `Cmd+N` |
+| Save current note | `Ctrl+S` / `Cmd+S` |
+| Open note finder | `Ctrl+P` / `Cmd+P` |
+| Toggle preview | `Ctrl+\\` / `Cmd+\\` |
+| Toggle theme | `Ctrl+Shift+T` |
+| Sign out | (header menu) |
+
+## Offline use
+
+librenotes works offline. The Progressive Web App caches the
+shell so you can open the page without a network, and your notes
+are mirrored into IndexedDB so they are readable and editable
+offline. Edits made offline are queued and pushed when you come
+back online; conflicts (when both you and another session edited
+the same note) prompt you to keep one version or the other.
+
+## Privacy
+
+Your notes are stored on disk per-tenant, isolated by the
+filesystem sandbox; no other user can access your directory. The
+session token is held in `sessionStorage` so closing the browser
+tab signs you out on shared devices. Per-user UI preferences live
+in `localStorage` under a tenant-scoped prefix and are wiped on
+logout.
+
+For the full privacy policy see [/privacy.html](/privacy.html).
+
+## Self-hosting
+
+Want to run your own instance instead? See
+[self-hosting.md](./self-hosting.md).