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/edge.textpb

@@ -0,0 +1,38 @@
+# Edge network with one transit provider and one session for each address
+# family. Route origin validation, ASPA verification, and kernel route
+# installation are enabled.
+router_id: "198.51.100.1"
+local_asn: 64500
+
+peers {
+  name: "transit-a-v4"
+  endpoint { address: "203.0.113.0" }
+  local_address: "203.0.113.1"
+  remote_asn: 64496
+  role: PROVIDER
+  address_families: IPV4_UNICAST
+  # Above the size of the IPv4 Internet routing table.
+  import { max_prefixes: 1200000 }
+}
+
+peers {
+  name: "transit-a-v6"
+  endpoint { address: "2001:db8:ffff:1::1" }
+  local_address: "2001:db8:ffff:1::2"
+  remote_asn: 64496
+  role: PROVIDER
+  address_families: IPV6_UNICAST
+  import { max_prefixes: 300000 }
+}
+
+originate { prefix: "198.51.100.0/24" }
+originate { prefix: "2001:db8:100::/40" }
+
+# RPKI cache on the local host, for example Routinator.
+rpki { caches { address: "::1" port: 3323 } }
+
+# Requires Linux and CAP_NET_ADMIN.
+kernel {}
+
+# Not authenticated. Bind to a loopback address.
+http_endpoint { address: "127.0.0.1" port: 8080 }