[Flang] Move markdown files(.MD) from documentation/ to docs/

Summary:
Other LLVM sub-projects use docs/ folder for documentation files.
Follow LLVM project policy.
Modify `documentation/` references in sources to `docs/`.
This patch doesn't modify files to reStructuredText(.rst) file format.

Reviewed By: DavidTruby, sscalpone

Differential Revision: https://reviews.llvm.org/D85884