[llvm] [docs] Do not require recommonmark for manpage build

Do not enforce recommonmark dependency if sphinx is called to build
manpages.  In order to do this, try to import recommonmark first
and do not configure it if it's not available.  Additionally, declare
a custom tags for the selected builder via CMake, and ignore
recommonmark import failure when 'man' target is used.

This will permit us to avoid the problematic recommonmark dependency
for the majority of Gentoo users that do not need to locally build
the complete documentation but want to have tool manpages.

Differential Revision: https://reviews.llvm.org/D83161