[libc][docs] start documenting c23 support (#81933)

I've been diffing the c17 vs c23 latest publicly available drafts and think I
have most of the library related differences. I haven't yet annotated what we
actually support or not.

Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf (C17)
Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf (C23)