I've moved the setting-up-docker bits into a separate page,
but have left it out the TOC. I'm not sure that level of
detail on docker belongs in this section; perhaps it could
move to an appendix?
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.
Mike Hillyer
Tom Mairs
Wez Furlong
Tom Mairs