Help: Add policy summaries to cmake-policies(7)

In Sphinx output formats that print the toctree the policy numbers in
links from the cmake-policies(7) manual are not descriptive.  Convert
the toctree entries to cross-reference syntax and add the summary of
each policy.  For now simply duplicate the policy summary line.  We
already maintain copies in `cmPolicies.h` and `Help/policy/*.rst` docs.