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/images/README.md

@@ -0,0 +1,8 @@
+# Images
+
+| File | Source | Licence | SHA-256 |
+|---|---|---|---|
+| `hero-2400.jpg`, `hero-1200.jpg` | "Fiber to the home" by Ronald Crow, https://unsplash.com/photos/a-bunch-of-yellow-and-green-wires-and-wires-8oR7XykOjk4, resized by images.unsplash.com to 2400 and 1200 pixels wide | [Unsplash License](https://unsplash.com/license): free to use; attribution not required, given on the front page | `c2bc75367507116c005e6c28773c18c4677a8255ac9887bdfe6c33de95caec27` (2400), `17712eb5701415d8faa8f5341ef145ddd17132c7bafccac033d789051bc59fa4` (1200) |
+
+The front page shows the photograph in two colours (see `.hero-photo` in
+`sass/site.scss`); the files themselves are the original colour.