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/syntaxes/textproto.json

@@ -0,0 +1,57 @@
+{
+  "name": "textproto",
+  "scopeName": "source.textproto",
+  "fileTypes": ["textproto", "textpb", "pbtxt"],
+  "patterns": [
+    { "include": "#comment" },
+    { "include": "#block" },
+    { "include": "#field" },
+    { "include": "#string" },
+    { "include": "#constant" },
+    { "include": "#number" }
+  ],
+  "repository": {
+    "comment": {
+      "name": "comment.line.number-sign.textproto",
+      "match": "#.*$"
+    },
+    "block": {
+      "comment": "A field whose value is a message: `peers {`",
+      "match": "\\b([A-Za-z_][A-Za-z0-9_]*)\\s*:?\\s*(?=\\{)",
+      "captures": { "1": { "name": "entity.name.tag.textproto" } }
+    },
+    "field": {
+      "match": "\\b([A-Za-z_][A-Za-z0-9_]*)\\s*(:)",
+      "captures": {
+        "1": { "name": "support.type.property-name.textproto" },
+        "2": { "name": "punctuation.separator.key-value.textproto" }
+      }
+    },
+    "string": {
+      "patterns": [
+        {
+          "name": "string.quoted.double.textproto",
+          "begin": "\"",
+          "end": "\"",
+          "patterns": [{ "name": "constant.character.escape.textproto", "match": "\\\\." }]
+        },
+        {
+          "name": "string.quoted.single.textproto",
+          "begin": "'",
+          "end": "'",
+          "patterns": [{ "name": "constant.character.escape.textproto", "match": "\\\\." }]
+        }
+      ]
+    },
+    "constant": {
+      "patterns": [
+        { "name": "constant.language.boolean.textproto", "match": "\\b(true|false)\\b" },
+        { "name": "constant.language.enum.textproto", "match": "\\b[A-Z][A-Z0-9_]*\\b" }
+      ]
+    },
+    "number": {
+      "name": "constant.numeric.textproto",
+      "match": "-?\\b(0x[0-9A-Fa-f]+|[0-9]+(\\.[0-9]+)?([eE][+-]?[0-9]+)?)\\b"
+    }
+  }
+}