feat: hold the site to page and index budgets

validate_site.py fails a page over 1 MB of HTML, a search index over
8.5 MB (1.7 MB gzip), a skipped heading level, a table header without
text, a script from another host and a diagram drawn at runtime. The
limits sit above what the site now measures (0.88 MB, 7.2 MB, 1.43 MB),
each with its reason, and each has a test.

wiki/release-process.md

@@ -54,7 +54,8 @@ is missing.
 6. Run `generate_readme.py` (rebuilds README.md and CONTRIBUTING.md)
 7. `mkdocs build --strict` to produce the static site
 8. Run `validate_site.py` on the rendered HTML (metadata, headings, image
-   alternatives, duplicate ids, local links and fragments)
+   alternatives, duplicate ids, local links and fragments, page and search
+   index budgets)
 9. Require the committed README and CONTRIBUTING to match what the generator
    just produced. `write_if_changed()` compares content with the timestamp line
    stripped, so a run that only moves the clock leaves the files untouched and