intersphinx issues in sunkit_instruments

[cbethge]

Description

The SunPy developer newcomer's guide says that intersphinx should work with Python, Numpy, and Astropy. I am guessing that for sunkit_instruments, it should also work with SunPy, as it is an affiliated package. However, when I write the restructured text Sphinx links in my code, only Numpy links are generated correctly, Astropy and SunPy links are not.

Expected vs Actual behavior

When using Sphinx links in Python code, e.g. `~numpy.ndarray` or `~sunpy.map.Map`, the expected behavior is that the links automatically point to the correct website in the documentation.

In sunkit_instruments, this only seems to work for Numpy (`~numpy.ndarray` for example gets compiled correctly), but not for e.g. `~sunpy.map.Map` or `astropy.io.fits.header.Header`.

Steps to Reproduce

See above.

System Details

• SunPy Version: 4.0.dev160+ga66cd925f
• Astropy Version: 5.0
• Python Version: 3.9.7
• OS information: Mac OS 10.16, Arch: 64bit, (i386)