User Manual should be in Markdown, not HTML
the HTML is hard to hand-maintain
one section of the User Manual is already in Markdown, and the build has support for doing the HTML conversion, so it would just need to be extended to other sections. it could happen a section at a time if redoing the whole manual at once is too daunting
pandoc is pretty good at converting markup formats. with a little luck, the results would only need minor manual cleanup