Broken GH links from gallery examples

[IoannisSifnaios]

When clicking on the GitHub link in the gallery examples, the link to the source code is incorrect. It seems that the GH link is missing the subfolder where the gallery example is placed. See the below example:

Documentation link: https://pvlib-python.readthedocs.io/en/stable/gallery/bifacial/plot_pvfactors_fixed_tilt.html#sphx-glr-gallery-bifacial-plot-pvfactors-fixed-tilt-py
Link to source code: https://github.com/pvlib/pvlib-python/blob/main/docs/examples/plot_pvfactors_fixed_tilt.py

[kandersolar]

Here is where that URL is determined: https://github.com/pvlib/pvlib-python/blob/main/docs/sphinx/source/conf.py#L443-L449

Looks like an oversight from when we moved the gallery pages into subdirectories! If so, that means those links have been broken for a few years, whoops...

I think it would be pretty straightforward to fix that code linked above to correctly account for the directory. A PR would be welcome!