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>

crates/server/tests/documentation.rs

@@ -0,0 +1,101 @@
+//! The configuration reference on the documentation site describes every
+//! field of the configuration's schema.
+//!
+//! The schema is `proto/config.proto`; the reference is
+//! `docs/content/docs/reference/configuration.md`, with a section for each
+//! message and enum (`## Peer`) that names each of its fields or values in
+//! backquotes. A field added to the schema and not to the reference fails
+//! here, so that operators can find every setting the daemon has.
+
+use std::collections::BTreeMap;
+use std::path::Path;
+
+/// The messages and enums of a `.proto` file, each with its fields or
+/// values, in the order they are declared.
+fn declarations(proto: &str) -> BTreeMap<String, Vec<String>> {
+    let mut found = BTreeMap::new();
+    let mut current: Option<(String, Vec<String>)> = None;
+    for line in proto.lines() {
+        let line = line.trim();
+        if line.starts_with("//") || line.is_empty() {
+            continue;
+        }
+        let words: Vec<&str> = line.split_whitespace().collect();
+        match words.as_slice() {
+            ["message" | "enum", name, "{"] => current = Some((name.to_string(), Vec::new())),
+            ["}"] => {
+                if let Some((name, members)) = current.take() {
+                    found.insert(name, members);
+                }
+            }
+            // `repeated Peer peers = 4;`, `string name = 1;`, `PEER = 3;`
+            _ => {
+                let Some((_, members)) = current.as_mut() else {
+                    continue;
+                };
+                let Some(at) = words.iter().position(|word| *word == "=") else {
+                    continue;
+                };
+                let name = words[at - 1];
+                // An enum's zero value is what is left out; there is
+                // nothing to configure.
+                if !name.ends_with("_UNSPECIFIED") {
+                    members.push(name.to_string());
+                }
+            }
+        }
+    }
+    found
+}
+
+/// The text of each `## ` section of a Markdown page, by its heading.
+fn sections(page: &str) -> BTreeMap<String, String> {
+    let mut found = BTreeMap::new();
+    let mut current: Option<(String, String)> = None;
+    for line in page.lines() {
+        if let Some(heading) = line.strip_prefix("## ") {
+            if let Some((name, text)) = current.take() {
+                found.insert(name, text);
+            }
+            current = Some((heading.trim().to_string(), String::new()));
+        } else if let Some((_, text)) = current.as_mut() {
+            text.push_str(line);
+            text.push('\n');
+        }
+    }
+    if let Some((name, text)) = current {
+        found.insert(name, text);
+    }
+    found
+}
+
+#[test]
+fn the_configuration_reference_has_every_field() {
+    let crate_dir = Path::new(env!("CARGO_MANIFEST_DIR"));
+    let proto = std::fs::read_to_string(crate_dir.join("proto/config.proto")).unwrap();
+    let reference_path = crate_dir.join("../../docs/content/docs/reference/configuration.md");
+    let reference = std::fs::read_to_string(&reference_path).unwrap();
+
+    let declared = declarations(&proto);
+    assert!(declared.contains_key("BgpConfig") && declared.contains_key("Peer"));
+    let documented = sections(&reference);
+
+    let mut missing = Vec::new();
+    for (name, members) in &declared {
+        let Some(section) = documented.get(name) else {
+            missing.push(format!("a section `## {name}`"));
+            continue;
+        };
+        for member in members {
+            if !section.contains(&format!("`{member}`")) {
+                missing.push(format!("`{member}` under `## {name}`"));
+            }
+        }
+    }
+    assert!(
+        missing.is_empty(),
+        "{} does not describe:\n  {}",
+        reference_path.display(),
+        missing.join("\n  ")
+    );
+}