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.

wiki/testing-guide.md

@@ -287,7 +287,23 @@ The test suite is one layer of verification. The full quality gate is:
 3. No unexpected CRITICAL entries in the verify output
 4. Pack file counts match verification file counts (consistency check)
 5. Pack integrity passes (every declared file extractable with correct hash)
-6. `mkdocs build --strict` and `python scripts/validate_site.py` pass
+6. `npm run build --prefix website` and `python scripts/validate_site.py` pass
+
+A change to the site engine, its version, its search ranking or the page
+weights is measured on a served build before it lands, with the scripts in
+`website/tools/` (they need `npm install --no-save playwright-core axe-core
+lighthouse` and Chrome):
+
+- `search_gate.py`: the queries a visitor types and the page that must answer
+  each, read from the site's own search box
+- `lighthouse.py`: Lighthouse mobile on a list of pages, the heaviest ones
+  included
+- `axe.mjs`: WCAG 2.2 AA in light and dark
+- `check_tools.mjs`: the hash lookup, the file picker, a hash typed in the
+  search, breadcrumbs, previous and next across tabs, glossary terms, tables
+  on a phone and the 404 page, in a real browser
+- `serve.mjs`: the build served under /retrobios/ with gzip, as GitHub Pages
+  serves it
 
 A change that passes the tests but breaks the pipeline is not merged. A new
 CRITICAL entry in the verify output after a change is explained before the
@@ -380,7 +396,7 @@ nav_title: Save files    # only when the menu names it otherwise than its headin
 - `level` picks the tab and the label shown at the top of the page.
 - `order` sorts the tab. Pages are spaced by ten, so a new page fits between
   two without renumbering. The generated pages (systems, emulators, the gaps
-  and data pages) hold fixed places among them, set in `generate_mkdocs_nav`.
+  and data pages) hold fixed places among them, set in `generate_nav`.
 - `nav_title` is the menu label. Without it the menu shows the page's `# `
   heading. It is not `title`, which both MkDocs and Astro Starlight read as
   the page's own title.
@@ -419,7 +435,7 @@ The `validate.yml` workflow runs `python -m unittest discover -s tests -t . -v`
 roads into main: every pull request, and every direct push, that touches
 `bios/`, `platforms/`, `emulators/`, `schemas/`, `scripts/`, `tests/`,
 `install.py` or an input of the site build such as `wiki/`. The same job then
-builds the site with `mkdocs build --strict`, so a page that would fail the
+builds the site with `npm run build --prefix website`, so a page that would fail the
 deploy fails the pull request. Work reaches main by push as often as by pull request, so a suite
 wired to pull requests alone would guard the road nobody takes. The test job
 (`run-tests`) runs in parallel with schema validation, and on a pull request