book: Publish the latest release instead of main

[nicholasbishop]

For #1222

• The book job now triggers on tag pushes in addition to pushes to main
• The job builds the book from the highest release tag instead of main
• The book output is added to the root of the gh-pages branch rather than a subdirectory
• Removed the now-unused redirect-to-HEAD html file

Did some testing in a fork to make sure the changes work as expected.

Note that the old HEAD folder will still be present on the gh-pages branch after running this, so it won't immediately break any links.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

Thanks! LGTM