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

We have a list of criteria for proposing an extension in Clang, but we
do not have any documentation about how to propose an extension. This
adds some basic documentation about how we run RFCs in Clang, as well as
adds a list of Clang Area Team members in support of the process.

It could be argued that this should be documented for the entire LLVM
Project, however, other parts of the project have different rules for
proposing extensions. (Extending libc++ is a different proposition than
extending LLVM, than extending MLIR, than extending Clang, etc.) We may
want to introduce high-level documentation for the LLVM Project, but
that is a bigger project and we already have the documentation in Clang
about criteria for extensions. So it seems reasonable to add some
lightweight documentation specific to Clang until we determine what to
do at the whole project level.

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