[libc++] Add coding guidelines to the docs (#117051)

We have a buch of coding guidelines which are either documented as
design docs, which aren't really applicable or not at all. This moves
coding guidelines we have currently in the design docs into a separate
file and adds a bunch of guidelines which we have but aren't documented
anywhere.

GitOrigin-RevId: 59716479fc2f78ccabb2fc47b23cdc636d4ce122
5 files changed