Help: Link to variables corresponding to id and version expressions (#15012)

In the cmake-generator-expressions(7) manual, link to the variables
that correspond to the PLATFORM_ID, C_COMPILER_ID, CXX_COMPILER_ID,
C_COMPILER_VERSION, and CXX_COMPILER_VERSION generator expressions.
Merge topic 'doc-genex-id-values' into next

108b62ba Help: Link to variables corresponding to id and version expressions (#15012)