[libc++][doc] Update the release notes.

I had a look at the changes since the last release and updated the
release notes with interesting changes.

It seems this time the release notes were already rather up to date :-)

If there are more interesting changes, please let me know and I'll
update the patch. I'd like to commit these changes latest next weekend
so they land before branching the 14.0 release.

I've added most active libc++ contributors. If I forgot anybody please add them.

Reviewed By: Quuxplusone, ldionne, philnik, #libc

Differential Revision: https://reviews.llvm.org/D117948