feat: build the site with Astro Starlight

generate_site.py writes the Starlight project in website/ and nothing
else: mkdocs.yml, the MkDocs hooks, theme overrides and stylesheet go,
and the wiki is written in Starlight's syntax. Both workflows set up
Node with the official actions/setup-node pinned by commit, run npm ci
from the committed lockfile, build with the strict link validator and
check the result with validate_site.py.

The search weighs a page by what it answers: Start here pages first,
a system page's cards and file names before a guide citing them, deep
guides, launchers, aliases and frozen copies after. Status badges,
hash blocks and the export change lists leave the index. The ranking
sits at the centre of a range where the 13 queries of search_gate.py
all answer, in the search box itself.

839 pages, every URL and 45,611 ids match the MkDocs build. The tests
read the Starlight content and syntax.

tests/sitepages.py

@@ -0,0 +1,46 @@
+"""The generated site pages, for the tests that read them.
+
+generate_site.py writes each page into the Starlight project's content
+collection, as .md or as .mdx when it uses a component. A test names a
+page by its docs path (``which-pack.md``) and gets whichever file holds it.
+"""
+
+from __future__ import annotations
+
+import os
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+CONTENT = REPO_ROOT / "website" / "src" / "content" / "docs"
+
+
+def require() -> Path:
+    """The content folder, or a skip when the site was not generated (a
+    failure in CI, which generates it before the suite)."""
+    if not (CONTENT / "index.md").is_file():
+        if os.environ.get("CI"):
+            raise AssertionError("CI generates the site before the suite")
+        raise unittest.SkipTest("site not generated (python scripts/generate_site.py)")
+    return CONTENT
+
+
+def page(name: str) -> Path:
+    """The file of a page named by its docs path."""
+    stem = name[:-3] if name.endswith(".md") else name
+    for suffix in (".md", ".mdx"):
+        candidate = CONTENT / f"{stem}{suffix}"
+        if candidate.is_file():
+            return candidate
+    return CONTENT / f"{stem}.md"
+
+
+def pages() -> list[Path]:
+    """Every page file, sorted."""
+    root = require()
+    return sorted(p for p in root.rglob("*") if p.suffix in (".md", ".mdx"))
+
+
+def name(path: Path) -> str:
+    """A page file's docs path, the name tests use for it."""
+    return path.relative_to(CONTENT).with_suffix(".md").as_posix()