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/base.html

@@ -0,0 +1,87 @@
+{%- import "macros/icons.html" as icons -%}
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>{% block title %}{{ config.title }}{% endblock title %}</title>
+  <meta name="description" content="{% block description %}{{ config.description }}{% endblock description %}">
+  <meta name="color-scheme" content="light dark">
+  <meta name="theme-color" media="(prefers-color-scheme: light)" content="#ffffff">
+  <meta name="theme-color" media="(prefers-color-scheme: dark)" content="#06101e">
+  <link rel="icon" href="{{ get_url(path='favicon.svg') }}" type="image/svg+xml">
+  <link rel="preload" href="{{ get_url(path='fonts/InterVariable.woff2') }}" as="font" type="font/woff2" crossorigin>
+  <link rel="stylesheet" href="{{ get_url(path='site.css', cachebust=true) }}">
+  {#- The theme is set before the page is drawn, so that it does not flash. #}
+  <script>
+    try {
+      var theme = localStorage.getItem("theme");
+      if (theme === "light" || theme === "dark") document.documentElement.dataset.theme = theme;
+    } catch (e) {}
+  </script>
+  <script src="{{ get_url(path='js/site.js', cachebust=true) }}" defer></script>
+</head>
+<body class="{% block body_class %}{% endblock body_class %}"
+      data-search-engine="{{ get_url(path='elasticlunr.min.js') }}"
+      data-search-index="{{ get_url(path='search_index.en.js') }}">
+  <a class="skip" href="#main">Skip to content</a>
+  <header class="topbar">
+    <div class="topbar-inner">
+      {%- block menu_button %}{% endblock menu_button %}
+      <a class="brand" href="{{ get_url(path='') }}" aria-label="Ferrix home">
+        <span class="brand-mark" aria-hidden="true"></span>
+        <span class="brand-name">Ferrix</span>
+        <span class="brand-version">{{ config.extra.version }}</span>
+      </a>
+      <nav class="topnav" aria-label="Main">
+        {%- for item in config.extra.nav %}
+        <a href="{{ get_url(path=item.path) }}"{% if current_path and current_path is starting_with("/" ~ item.path) %} aria-current="page"{% endif %}>{{ item.name }}</a>
+        {%- endfor %}
+      </nav>
+      <div class="topbar-actions">
+        <button class="search-button" type="button" data-search-open aria-label="Search the documentation">
+          {{ icons::search() }}
+          <span class="search-button-label">Search</span>
+          <kbd class="search-button-key">/</kbd>
+        </button>
+        <a class="icon-button" href="{{ config.extra.repository }}" aria-label="Source code">{{ icons::code() }}</a>
+        <button class="icon-button" type="button" data-theme-toggle aria-label="Switch between light and dark">
+          <span class="theme-icon-light">{{ icons::sun() }}</span>
+          <span class="theme-icon-dark">{{ icons::moon() }}</span>
+        </button>
+      </div>
+    </div>
+  </header>
+
+  {% block body %}{% endblock body %}
+
+  <footer class="footer">
+    <div class="footer-inner">
+      <div class="footer-brand">
+        <span class="brand-mark" aria-hidden="true"></span>
+        <span>Ferrix BGP {{ config.extra.version }}</span>
+      </div>
+      <nav class="footer-links" aria-label="Footer">
+        <a href="{{ get_url(path='docs/getting-started/introduction/') }}">Documentation</a>
+        <a href="{{ get_url(path='docs/reference/configuration/') }}">Configuration</a>
+        <a href="{{ get_url(path='docs/reference/rfcs/') }}">RFCs</a>
+        <a href="{{ config.extra.repository }}">Source</a>
+        <span>Apache License 2.0</span>
+      </nav>
+    </div>
+  </footer>
+
+  <div class="search" data-search hidden>
+    <div class="search-backdrop" data-search-close></div>
+    <div class="search-dialog" role="dialog" aria-modal="true" aria-label="Search">
+      <div class="search-field">
+        {{ icons::search() }}
+        <input type="search" placeholder="Search the documentation" autocomplete="off" spellcheck="false" data-search-input aria-label="Search">
+        <kbd data-search-close>esc</kbd>
+      </div>
+      <ul class="search-results" data-search-results role="listbox"></ul>
+      <p class="search-empty" data-search-empty hidden>Nothing found.</p>
+    </div>
+  </div>
+</body>
+</html>