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/macros/icons.html

@@ -0,0 +1,14 @@
+{#- Icons, drawn on a 24 by 24 grid with a 1.75-pixel stroke in the current
+    colour: square ends, as the type is. -#}
+
+{%- macro svg_open() -%}
+<svg class="icon" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.75" stroke-linecap="square" aria-hidden="true">
+{%- endmacro svg_open -%}
+
+{%- macro search() -%}{{ self::svg_open() }}<circle cx="11" cy="11" r="6.5"/><path d="m16 16 4.5 4.5"/></svg>{%- endmacro search -%}
+{%- macro code() -%}{{ self::svg_open() }}<path d="m16 18 6-6-6-6"/><path d="m8 6-6 6 6 6"/></svg>{%- endmacro code -%}
+{%- macro sun() -%}{{ self::svg_open() }}<circle cx="12" cy="12" r="4"/><path d="M12 2v2M12 20v2M4.9 4.9l1.4 1.4M17.7 17.7l1.4 1.4M2 12h2M20 12h2M4.9 19.1l1.4-1.4M17.7 6.3l1.4-1.4"/></svg>{%- endmacro sun -%}
+{%- macro moon() -%}{{ self::svg_open() }}<path d="M20.5 13.5A8.5 8.5 0 1 1 10.5 3.5a6.6 6.6 0 0 0 10 10z"/></svg>{%- endmacro moon -%}
+{%- macro menu() -%}{{ self::svg_open() }}<path d="M4 7h16M4 12h16M4 17h16"/></svg>{%- endmacro menu -%}
+{%- macro arrow_right() -%}{{ self::svg_open() }}<path d="M4 12h15M13 6l6 6-6 6"/></svg>{%- endmacro arrow_right -%}
+{%- macro file() -%}{{ self::svg_open() }}<path d="M14 3H6v18h12V7z"/><path d="M14 3v4h4"/></svg>{%- endmacro file -%}