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/templates/shortcodes/file.html

@@ -0,0 +1,8 @@
+{#- A file from docs/examples, shown whole and highlighted as `lang`. -#}
+{%- set text = load_data(path="examples/" ~ name, format="plain") -%}
+{%- set fenced = "```" ~ lang ~ "
+" ~ text ~ "```" -%}
+<figure class="example">
+<figcaption>docs/examples/{{ name }}</figcaption>
+{{ fenced | markdown | safe }}
+</figure>