[LLDB] Update docs on building documentation (#69858)

This patch updates the documentation to match recent changes and make it
more clear. More specifically, the process for installing sphinx has
changed with the transition to myst with the requirements.txt in
llvm/docs being the preferred method for installation now. In addition,
the docs-lldb-html target is never generated if swig isn't installed, so
having something expliti in the documentation section (even if it is
mentioned as a dependency of lldb itself above) probably doesn't hurt.