Update the type comparison for C++23 expected

[ac1982]

Fixes #85.

Update the comparison for C++23 std::expected, replacing the experimental-namespace note with the standard <expected> API and a link to the C++23 draft. Clarify that the bundled nonstd::expected column applies when the fallback is explicitly selected.

The comparison now distinguishes equality from ordering, shows the default-construction constraint, and updates the adjacent standard-library reference, piecewise-construction and monadic-operation entries. The new C++23 row does not assume compatibility for the older third-party implementations.

Validation: compiled and ran a C++23 probe with GCC 15.2 for both the standard selection and forced bundled implementation; checked construction constraints, equality/no ordering, pair references and piecewise construction, expected's four monadic operations, and optional chaining. Confirmed C++23 reference-valued optional/expected instantiations are rejected. Markdown rendering retains nine columns per row; git diff --check passes. No library code changed, so the full compiler test matrix was not run.

Prepared with Codex assistance; statements checked against the C++23 draft and current header.

[martinmoene]

Thanks @ac1982