CMAKE_COMPILER_IS_*: Add RST deprecation directives and update docs

The `CMAKE_COMPILER_IS_*` variables have been documented as deprecated
since CMake 3.24, without emitting warnings.  This commit updates their
documentation to help users safely migrate to `CMAKE_<LANG>_COMPILER_ID`
variables. Also a RST deprecation directive is added to clarify their
status.
3 files changed