CXX-3069 document headers and root namespace redeclarations (mongocxx)

[eramongodb]

Resolves CXX-3069. Followup to #1182.

Completes the addition of documentation of files and root namespace redeclarations for mongocxx using the same pattern as that for bsoncxx.

A couple changes beyond the regular include:

• Added missing docs for mongocxx::stdx and mongocxx::v_noabi::stdx, which have been marked as deprecated in favor of directly using bsoncxx::stdx instead.
• Updated deprecation notes for mongocxx::v_noabi::create_collection and mongocxx::v_noabi::ssl.
• Added missing docs for some interfaces to resolve @ref links (e.g. validation_criteria comparison operators, typedefs and type aliases, etc.).