[libcxx][spaceship][doc] Repair links and clean up spaceship progress doc

Previously the specification was in a table in paragraph 14 of [container.requirements.general] but it has since been given its own sub section header, 24.2.2.4 [container.opt.reqmts].

This happened in https://github.com/cplusplus/draft/commit/93ff092d1cd2b335f372b9546365b3d495caf2d8

This update also includes misc. other cleanups of the spaceship projects sheet.

Reviewed By: Mordante, avogelsgesang, #libc

Differential Revision: https://reviews.llvm.org/D131928