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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

These changes look reasonable to me, but I'd appreciate a final sign off from @mgorny to make sure this works for his needs.

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