DOC: Add examples for (most) user-facing functions [pyOpenSci review]

[NickleDave]

The pyOpenSci review requires examples "for all user-facing functions".

My impression reading the API docs is that there are some examples, e.g. for the load method of the plenoptic.synthesize.Metamer class:
https://plenoptic.readthedocs.io/en/latest/api/plenoptic.synthesize.html#plenoptic.synthesize.metamer.Metamer.load

But that there are some frequently used methods and functions that do not have examples, e.g.

• https://plenoptic.readthedocs.io/en/latest/api/plenoptic.synthesize.html#plenoptic.synthesize.metamer.Metamer.synthesize
• https://plenoptic.readthedocs.io/en/latest/api/plenoptic.tools.html#plenoptic.tools.validate.remove_grad

I know that adding an example for every single solitary user-facing function can get quite tedious, so I would use your best judgement, but it seems like there should examples for most functions, at least for the core functionality.

Probably you will want to add built-in data first as in #235 and then leverage that in the examples.
Looks like you are already set up to use your plots in the docs? https://github.com/LabForComputationalVision/plenoptic/blob/de8f9308d0fcd24050a93b326390eb4444b9aff2/docs/conf.py#L48.
But you might need to do more set up to get images in rendered docstrings, like
https://github.com/librosa/librosa/blob/8dae20c66353190749486abf6b332d93c4ab2b46/docs/conf.py#L107C1-L113C4

[billbrod]

When I've done this, should also use pytest to run doctests, testing these examples.