PEP 12: Add list of intersphinx mappings

[zklaus]

PEP 12 is very helpful in writing PEPs and has a section on intersphinx. However, to know the available intersphinx mappings, one has to know where to look in the sphinx configuration.

I think it would be useful to include the list directly in PEP 12.

Alternatively, if that is considered to be at too high a risk for getting outdated, one could add an explanation on where to find it.