[Docs] Allow building man pages without myst_parser (#82402)

The man pages do not depend on the doc present in markdown files, so
they can be built without myst_parser.
Doing so might allow llvm distributions to have less development
dependencies.

As we do not have the ennvironment to test these configuration, this
capability is provided on a best-effort basis.

This restores a capability accidentally lost in #65664.