Markdown support in help section

[bgruening]

Galaxy 23.2 will ship with markdown support in the tool help section. Would be nice if we could support that and encourage users to use it.

• support markdown
• try to convert rst to markdown, especially if the profile version is >= 23.2
• support the new fancy <HelpText text="mapped over" uri="galaxy.collections.mapOver" /> syntax
  • find those terms may be automatically and offer a conversion to this new syntax? We could use the new API (/help/terms/{term_id})

xref: galaxyproject/galaxy#18722