Add documentation and tests for setup_helpers.pkg_config

[astrofrog]

The setup_helpers.pkg_config function is used in several of astropy's setup_package.py files, so we should keep it here, but it isn't tested or documented, so that will need to be fixed.

[saimn]

The function is simple enough, so I'm not sure if it is worth changing, but there is a python package wrapping pkg-config: https://github.com/matze/pkgconfig
I saw it used by some package I don't remember. It is a single file so it could be bundled, which would also remove the need to test and document our version.

[astrofrog]

Oh interesting! Actually if we want to go down that road I suggest we just remove that from extension-helpers and have packages add pkgconfig to their pyproject.toml and use it directly. Is there any reason not to do that?

[saimn]

Oh yes indeed, that would be even simpler.

[Cadair]

👍 for removing everything we can 😀

[astrofrog]

I won't have time to investigate this in the near future, but if one of you has time to take a look, feel free to open a PR to astropy core to demonstrate how we could use the pkgconfig package instead!