Allow referencing/linking to individual fields in pyproject.toml

[abravalheri]

I was trying to right some documentation for setuptools that reference a couple of new fields in the pyproject.toml PyPUG document using intersphinx.

Initially I tried to use a fragment in the :doc: role like the following:

:doc:`project.license <PyPUG:specifications/pyproject-toml/#license-files>`
:doc:`project.license <PyPUG:specifications/pyproject-toml/#license>`

But it seems that Sphinx doesn't know how to handle fragments (If I remove the fragment everything is fine):

/home/runner/work/setuptools/setuptools/docs/userguide/pyproject_config.rst:74: WARNING: unknown document: 'PyPUG:pecifications/pyproject-toml/#license-files' [ref.doc]
/home/runner/work/setuptools/setuptools/docs/userguide/pyproject_config.rst:74: WARNING: unknown document: 'PyPUG:pecifications/pyproject-toml/#license' [ref.doc]

This means that in order link to an individual field in the PyPUG document, it would be necessary to add reference anchors in the PyPUG rst file first.

This is what this PR is trying to achieve.

When the references are in place, than I could use something like the following:

:external+PyPUG:ref:`project.license <pyproject-toml-license>

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📚 Documentation preview 📚: https://python-packaging-user-guide--1832.org.readthedocs.build/en/1832/

[webknjaz]

Initially I tried to use a fragment in the :doc: role like the following:

The :doc: role is for documents, as in entire web pages. Declaring refs would work. I have a little helper for myself where you can see what objects are linkable: https://webknjaz.github.io/intersphinx-untangled/packaging.python.org/.

[webknjaz]

thanks!

[abravalheri]

Thank you very much @webknjaz