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/section.html

@@ -0,0 +1,61 @@
+{% extends "base.html" %}
+{%- import "macros/icons.html" as icons -%}
+{%- import "macros/docs.html" as docs -%}
+
+{% block title %}{{ section.title }} · {{ config.title }}{% endblock title %}
+{% block description %}{{ section.description | default(value=config.description) }}{% endblock description %}
+{% block body_class %}layout-docs{% endblock body_class %}
+
+{% block menu_button %}
+<button class="icon-button menu-button" type="button" data-menu-toggle aria-label="Show the contents" aria-expanded="false">{{ icons::menu() }}</button>
+{% endblock menu_button %}
+
+{% block body %}
+<div class="docs-layout">
+  <aside class="sidebar" data-sidebar>
+    {{ docs::sidebar(current=section.path) }}
+  </aside>
+  <main id="main" class="docs-main">
+    <article class="prose">
+      <header class="page-header">
+        <span class="eyebrow">{% if section.extra.number %}{{ section.extra.number }} — {% endif %}Documentation</span>
+        <h1>{{ section.title }}</h1>
+        {%- if section.description %}
+        <p class="lead">{{ section.description }}</p>
+        {%- endif %}
+      </header>
+      {{ section.content | safe }}
+    </article>
+    {#- A group lists its pages; the top of the documentation lists the groups. #}
+    {%- if section.pages | length > 0 %}
+    <div class="cards">
+      {%- for p in section.pages %}
+      <a class="card" href="{{ p.permalink }}">
+        <span class="card-number">{{ section.extra.number }}.{{ loop.index }}</span>
+        <span class="card-title">{{ p.title }} {{ icons::arrow_right() }}</span>
+        {%- if p.description %}<span class="card-text">{{ p.description }}</span>{% endif %}
+      </a>
+      {%- endfor %}
+    </div>
+    {%- else %}
+    {%- for sub_path in section.subsections %}
+    {%- set sub = get_section(path=sub_path) %}
+    <section class="group-overview">
+      <h2><span>{{ sub.extra.number }}</span><a href="{{ sub.permalink }}">{{ sub.title }}</a></h2>
+      {%- if sub.description %}<p>{{ sub.description }}</p>{% endif %}
+      <div class="cards">
+        {%- for p in sub.pages %}
+        <a class="card" href="{{ p.permalink }}">
+          <span class="card-number">{{ sub.extra.number }}.{{ loop.index }}</span>
+          <span class="card-title">{{ p.title }} {{ icons::arrow_right() }}</span>
+          {%- if p.description %}<span class="card-text">{{ p.description }}</span>{% endif %}
+        </a>
+        {%- endfor %}
+      </div>
+    </section>
+    {%- endfor %}
+    {%- endif %}
+  </main>
+  <aside class="toc-column"></aside>
+</div>
+{% endblock body %}