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/examples/transit.textpb

@@ -0,0 +1,70 @@
+# Transit network with two providers and one customer. Customer routes are
+# advertised to all neighbors. Provider routes are advertised to the
+# customer only. The BGP roles of the neighbors enforce this.
+router_id: "198.51.100.1"
+local_asn: 64500
+
+# Advertise learned routes to all neighbors, as BGP roles permit.
+export { learned_routes: true }
+
+# Customers can restrict the advertisement of their routes with the large
+# communities 64500:0:P (not to AS P), 64500:1:P (only to AS P), and
+# 64500:0:0 (to no neighbor), or the standard communities 0:P, 64500:P, and
+# 0:64500.
+control_communities: true
+
+peers {
+  name: "transit-a"
+  endpoint { address: "203.0.113.0" }
+  remote_asn: 64496
+  role: PROVIDER
+  import {
+    max_prefixes: 1500000
+    remove_communities: "64500:2:*"
+    add_large_communities: "64500:2:100"
+  }
+}
+
+peers {
+  name: "transit-b"
+  endpoint { address: "203.0.113.2" }
+  remote_asn: 64497
+  role: PROVIDER
+  import {
+    max_prefixes: 1500000
+    remove_communities: "64500:2:*"
+    add_large_communities: "64500:2:200"
+  }
+  # Prepend to make the path through transit-b less preferred. This
+  # statement replaces the top-level export statement for this neighbor.
+  export { learned_routes: true as_path_prepend: 2 }
+}
+
+# Customer with a four-octet AS number. The prefix and origin AS lists
+# restrict its routes to its registered address space. Local preference
+# 200 makes its routes preferred over the same prefixes from providers.
+peers {
+  name: "customer-65536"
+  endpoint { address: "203.0.113.5" }
+  remote_asn: 65536
+  role: CUSTOMER
+  require_role: true
+  import {
+    max_prefixes: 50
+    prefixes { prefix: "192.0.2.0/24" }
+    prefixes { prefix: "2001:db8:a00::/40" max_length: 48 }
+    origin_asns: 65536
+    remove_communities: "64500:2:*"
+    add_large_communities: "64500:2:300"
+    local_pref: 200
+  }
+}
+
+originate { prefix: "198.51.100.0/24" }
+originate { prefix: "2001:db8:100::/40" }
+
+rpki { caches { address: "::1" port: 3323 } }
+
+kernel {}
+
+http_endpoint { address: "127.0.0.1" port: 8080 }