Spec: add paramspec reference to link from PEP 612

[hugovk]

This will allow referencing via intersphinx from other Sphinx sites, specifically from PEP 612.

I'm about to update python/peps#3575 so that it can use this to refer to here as the canonical documentation.

This is how the PyPA packaging specs do it, for example:

• https://peps.python.org/pep-0376/
• https://github.com/python/peps/blob/9498cb7107bcb14e816fbf533ca6bd968aedd953/peps/pep-0376.rst?plain=1#L13
• https://github.com/pypa/packaging.python.org/blob/8079afc6fc3bca0135df3646b97dbae1e7ab00af/source/specifications/core-metadata.rst?plain=1#L3

[srittau]

I'm not sure how the council wants to handle reviews and merges of editorial changes by non-council members in this repo, but this LGTM.

[JelleZijlstra]

@srittau feel free to merge this sort of change that doesn't impact content. If somebody on the Council disagrees, we can always discuss it later.