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

[Reid Kleckner via llvm-commits]

rnk 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?

Sure. I updated the PR with two git commits, but I think I will manually push them together (I believe we still have the ability to do that, with warnings) so that we don't have a window of broken / misformatted documentation. I confirmed the blame works locally, it goes back to the old code owners file now.

> Have you verified this actually builds the correct documentation? 

Yes, I built the `docs-clang-html` target and verified that the output is *almost* identical. It has one extra heading:
<img width="2049" height="1289" alt="Screenshot 2026-02-10 at 9 14 44 AM" src="https://github.com/user-attachments/assets/8f43ec75-cac9-41df-bd7d-d80efddce319" />


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