Fix `docs/conf.py` packaging issues

[mattwthompson]

Many projects are affected by what openforcefield/openff-interchange#1329 fixed, so I'm going to roll out changes everywhere I see the same behavior.

Cases can be found by paying attention when creating environments as

$ micromamba create --name foo openff-toolkit --yes | grep "following files"
warning  libmamba [xorg-libx11-1.8.12-h6a5fb8c_0] The following files were already present in the environment:
    - include/X11/Xlib.h
    - include/X11/Xutil.h
    - include/X11/cursorfont.h
warning  libmamba [openff-nagl-models-2025.9.0-pyhd8ed1ab_0] The following files were already present in the environment:
    - lib/python3.13/site-packages/docs/conf.py
warning  libmamba [openff-units-0.3.1-pyhd8ed1ab_2] The following files were already present in the environment:
    - lib/python3.13/site-packages/docs/conf.py
warning  libmamba [openff-toolkit-base-0.18.0-pyhd8ed1ab_0] The following files were already present in the environment:
    - lib/python3.13/site-packages/docs/conf.py
warning  libmamba [openff-interchange-base-0.4.9-pyhd8ed1ab_0] The following files were already present in the environment:
    - lib/python3.13/site-packages/docs/conf.py