[LLVM][Maintainers] Move code owner list to Maintainers.md (#110240)

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

The general format of the Maintainers.md file follows the current
CodeOwners.rst from clang, with some minor terminology 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 The list is still very badly outdated.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get proper diffs when replacing no longer active maintainers.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