For#1222

• The book job now triggers on tag pushes in addition to pushes tomain
• The job builds the book from the highest release tag instead ofmain
• The book output is added to the root of thegh-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 oldHEAD folder will still be present on thegh-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 theRewriting History guide for help.
• Update the changelog (if necessary)