[clang] [flang] [llvm] [Docs] Allow building man pages without myst_parser (PR #82402)

[Michał Górny via cfe-commits]

https://github.com/mgorny approved this pull request.

Thank you. From a quick test run with and without `myst-parser`, this seems to work as intended.

https://github.com/llvm/llvm-project/pull/82402