Ideally there would be a good mkdocs based checker, but the
one that is out there only validate absolute URLs, which makes
it unsuitable for checking local relative links in the docs.
Adjust the doc build so that we also generate the SUMMARY.md
for mdbook, and use mdbook's link checker to check the links.
Fixup the couple of broken links that it found.
the pages workflow silently corrects these so they go unnoticed,
leaving the corrections to show up for the next person to run
`docs/build.sh`, polluting their later commit with unrelated
changes.
Augment the verify-pages workflow to also run on `main` and
surface the corrections with a failed workflow run.
Wez Furlong