[Flang] man pages cannot be built

[pawosm-arm]

I'm trying to build man pages along with LLVM, clang and flang with CMake options as such:

    -DLLVM_BUILD_DOCS=ON \
    -DLLVM_ENABLE_SPHINX=ON \
    -DLLVM_ENABLE_PROJECTS="llvm;clang;flang;lld" \
    -DLLVM_ENABLE_RUNTIMES="compiler-rt;flang-rt;libunwind;openmp"

It fails like this:

FAILED: tools/flang/docs/CMakeFiles/docs-flang-man build/tools/flang/docs/CMakeFiles/docs-flang-man
cd build/tools/flang/docs && /usr/bin/cmake -E env ~/.local/bin/sphinx-build -b man -d build/tools/flang/docs/_doctrees-flang-man -q -t builder-man -D version=21 -D release=21 -W flang/docs build/tools/flang/docs/man
WARNING: no "man_pages" config value found; no manual pages will be written
flang/docs/index.md:40: WARNING: toctree contains reference to nonexisting document 'FIRLangRef' [toc.not_readable]
flang/docs/index.md:40: WARNING: toctree contains reference to nonexisting document 'FlangCommandLineReference' [toc.not_readable]

The build does not fail when I add the following line, thought it won't build flang manpages:

  -DFLANG_INCLUDE_DOCS=OFF

[snarang181]

Hello, I have linked a PR which fixes this issue. Please review.