Utilities/Release: Install docs in unversioned directory (#15692)
Our upstream-built release tarballs already contain many unversioned paths and so will never overlap with another version of CMake in the same installation prefix. Therefore we do not need a versioned name for the documentation directory. Configure our release binaries to place the documentation in an unversioned directory so that one can use the same path to refer to the documentation locally even after updating CMake. For example, on OS X one may see the documentation in `/Applications/CMake.app/Contents/doc/cmake/html/index.html`.