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>

.github/workflows/ci.yml

@@ -31,15 +31,35 @@ jobs:
         env:
           RUSTDOCFLAGS: -D warnings
         run: cargo doc --locked --workspace --no-deps --document-private-items
-      # The example operators start from, through the daemon itself.
-      - name: Example configuration
-        run: cargo run --locked -p bgp -- --check testdata/server.textpb
+      # The examples operators start from, here and in the documentation,
+      # through the daemon itself.
+      - name: Example configurations
+        run: |
+          for config in testdata/server.textpb docs/examples/*.textpb; do
+            cargo run --locked -p bgp -- --check "$config"
+          done
       # The tests that change a kernel's routes, in a network namespace of
       # their own; and two daemons that route for each other.
       - name: Kernel tests
         run: scripts/kernel-tests.sh
 
-  # The daemon against FRR, BIRD and Routinator; see bin/bgp/README.md.
+  # The documentation site. Zola fails the build on a link to a page or an
+  # anchor that is not there.
+  docs:
+    runs-on: ubuntu-latest
+    env:
+      ZOLA_VERSION: "0.22.1"
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Zola
+        run: |
+          curl -sSfL "https://github.com/getzola/zola/releases/download/v${ZOLA_VERSION}/zola-v${ZOLA_VERSION}-x86_64-unknown-linux-gnu.tar.gz" \
+            | sudo tar -xz -C /usr/local/bin zola
+      - name: Build
+        run: zola --root docs build
+
+  # The daemon against FRR, BIRD and Routinator; see
+  # docs/content/docs/development/testing.md.
   interop:
     runs-on: ubuntu-latest
     steps: