[libc++][ranges][NFC] Revamp the Ranges status page

Focus on the not-yet-implemented features: remove most details about the
already-implemented C++20 stuff, list out the major C++23 additions.

Differential Revision: https://reviews.llvm.org/D136657

GitOrigin-RevId: 2c5a548b53434496ac67a24f19543cf0a5a3ed41
10 files changed