[LLVM][Maintainers] Move code owner list to Maintainers.rst

Following https://discourse.llvm.org/t/rfc-proposing-changes-to-the-community-code-ownership-policy/80714 and llvm#107384,
this PR moves the llvm CODE_OWNERS.txt to the Maintainers.rst file.

The format of the Maintainers.rst file follows the current
CodeOwners.rst from clang, with some minor changes to account
for the code owners -> maintainers migration.

This change is intended to be a pure format change, without
changing any actual code owners / maintainers. The list is still
very badly outdated. The intent here is to get the format change
out of the way, so we can get proper diffs when replacing no longer
active maintainers. If we mix both together, it will be hard to
follow what actually changed.

I've tried to categorize things as best as I could, and filled in
GitHub handles for people where I knew them or could determine them
with some confidence.

Author: nikic