[libcxx-commits] [libcxx] [libc++][docs] Rename remaining rst docs to md [1/2] (PR #233994)

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s4chinjha wrote:

@philnik777 thanks for letting me know, i wasn't aware of that!
i'll drop the `ReleaseNotes/` files from the pr and only convert the ReleaseNotesTemplate to md

https://github.com/llvm/llvm-project/pull/233994