[clang] [Docs] Explain how to propose an extension in Clang (PR #130803)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Eventually, I expect we'll add documentation about the Clang Area Team to `clang/docs` as .rst files. Once that happens, I think we should switch the `AreaTeamMembers.txt` file to a .rst file and then link to the rendered content from `get_involved.html`. But I figured that can be done in follow up work.

https://github.com/llvm/llvm-project/pull/130803