[libc++] [docs] Add C++2b (to be C++23) status page.

[curdeius]

Summary

Also:

• Fix header line in all status tables.
• Use C++20 instead of C++2a.

[miscco]

Awesome, thanks a lot!

[Mordante]

Thanks for the patch.
One minor nit, other than that LGTM.

libcxx/docs/Cxx2aStatus.rst

18

Can you add a line mentioning the standard was approved in September 2020?
(https://herbsutter.com/2020/09/06/c20-approved-c23-meetings-and-schedule-update/)

[ldionne]

LGTM with @Mordante 's suggestion. Thanks!