Typeset user manual using TeX4ht rather than Pandoc

[Witiko]

Currently, the Markdown package uses Pandoc to typeset the HTML user manual. Replacing Pandoc with the Markdown package via TeX4ht as discussed in https://github.com/Witiko/markdown/issues/90#issuecomment-1031381574 makes us self-sufficient and allows us to leverage the reflection capabilities of the package to automatically generate parts of the manual as discussed in https://github.com/Witiko/markdown/discussions/135.