feat: Better support for cross-compilation

[LecrisUT]

Mostly the cross-compilation works out-of-the-box with crossenv + CMAKE_TOOLCHAIN_FILE. There is only one adjustment needed for Python_ROOT_DIR to point to sys.base_exec_prefix, the rest is documentation of the change and adding support for CMAKE_TOOLCHAIN_FILE where needed.

I presume that under PEP720 the same workflow will be valid, primarily sys.base_exec_prefix would point to the target's implementation, otherwise the document should be clarified on what to expect. I have raised the issue for further clarification ^1