Add a documentation website, built with Zola

The documentation moves from READMEs into a site in docs/: getting
started, a user guide, operations, a reference, and development pages,
written in the style of network vendors' documentation (docs/STYLE.md).
bin/bgp/README.md and docs/RFCs.md are folded into it.

- A theme of its own in the International Typographic Style, in blues,
  with self-hosted Inter and JetBrains Mono, search over Zola's index,
  dark mode, and no resources loaded from other hosts.
- Example configurations in docs/examples/, shown on the pages and
  checked with `bgp --check` in CI.
- A test that fails when a field of config.proto is missing from the
  configuration reference.
- A CI job that builds the site; Zola fails on broken links and anchors.
- A hardened systemd unit, tried under systemd with kernel routes on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/static/fonts/README.md

@@ -0,0 +1,13 @@
+# Fonts
+
+Served with the site, so that no page loads anything from elsewhere. Both
+are under the SIL Open Font License 1.1; the licences are beside them.
+
+| File | From | SHA-256 |
+|---|---|---|
+| `InterVariable.woff2` | Inter 4.1, `web/` in https://github.com/rsms/inter/releases/download/v4.1/Inter-4.1.zip | `693b77d4f32ee9b8bfc995589b5fad5e99adf2832738661f5402f9978429a8e3` |
+| `JetBrainsMono-Regular.woff2` | JetBrains Mono 2.304, `fonts/webfonts/` in https://github.com/JetBrains/JetBrainsMono/releases/download/v2.304/JetBrainsMono-2.304.zip | `a9cb1cd82332b23a47e3a1239d25d13c86d16c4220695e34b243effa999f45f2` |
+| `JetBrainsMono-Medium.woff2` | the same | `086c48dfbea9ddaff1320f7e09399b8e2924e88ce67453721255db3bdbb5a353` |
+
+Inter is variable in weight and in optical size: large headings get its
+display cut, with tighter spacing and finer detail, without a second font.