[clang] [docs] Convert Maintainers.rst to Markdown (.md) (PR #180652)

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

Perhaps it would be better to do this in two seperate patches, so that Git identifies this as a renaming and doesn’t lose history?

https://github.com/llvm/llvm-project/pull/180652