Document C++20 module usage with CMake

[way4sahil]

Adds CMake guidance for using {fmt} as a C++20 module.

• Enable FMT_MODULE before adding {fmt} as a subdirectory
• Link against fmt::fmt
• Use import fmt; in consumer source files
• Document the CMake 3.28+ and Ninja requirement for GCC module dependency scanning

Fixes #4237.

[vitaut]

Thanks for the PR.

The documented example can fail because fmt::fmt-module is only defined when FMT_MODULE is enabled. Could you mention that FMT_MODULE needs to be enabled before adding fmt (or otherwise ensure modules are enabled via CMAKE_CXX_STANDARD)? This is already mentioned in the PR description but not in the documentation itself.

Also, the GCC/CMake/Ninja versions listed here seem to be requirements for CMake’s native module support rather than for fmt::fmt-module in general, since fmt also has the fallback path.

[vitaut]

There is extra \ here.