[FLANG] Pick `.md` files when building sphinx documentation.

Need to build sphinx using below flags to Cmake
`-DLLVM_ENABLE_SPHINX=ON -DSPHINX_WARNINGS_AS_ERRORS=OFF`.
Generate html docs using cmake target
`docs-flang-html`
Generated html files should be at `build/tools/flang/docs/html`.
Patch in series from the dicussion on review
https://reviews.llvm.org/D85828

After this patch the markdown docmentation must be written using guide in-
`llvm/docs/MarkdownQuickstartTemplate.md`

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D86131