[libc++] Make sure to publish release notes at ReleaseNotes.html

In LLVM 17, we switched to numbered RST files for release notes, which
makes it easier to deal with cherry-picks around release points. However,
we stopped publishing `libcxx/docs/ReleaseNotes.html`, which was
referenced by external sites.

This patch ensures that we keep publishing `ReleaseNotes.html` by simply
including the versioned RST file in the unversioned RST file.

Fixes #77955