MAINT: Make selenium optional and use on CircleCI

[larsoner]

See title, should make it easier to build our docs. @britta-wstnr can you try building docs on this branch and see if it works? I made the avatar image gen opt-in (and opted CircleCI in) rather than opt-out so you should just be able to check out my branch, and run make clean then make html and see if it succeeds. I think we should iterate until things work for you to improve our odds of other contributors being able to build docs.

[larsoner]

Will fail until #12260 lands but local builds now show this instead of the contributor image:

[britta-wstnr]

Thanks @larsoner !
Running make html-pattern ... works for me again ! 🎉

I also tried running a full make html but this throws a memory error for me (at 35_dipole_orientations.py, which ultimately results in pyvista.plotting.errors.RenderWindowUnavailable: Render window is not current.and then sphinx.errors.ExtensionError: Handler <function generate_gallery_rst at 0x16997afc0> for event 'builder-inited' threw an exception (exception: [<mne.viz._brain._brain.Brain object at 0x2a8f0e510>])).
This surprises me a bit, as I have 16 GB RAM. I will try again tonight after closing everything else down.

[larsoner]

Up to date pyvista and pyvistaqt?

I think having the pattern working is enough for this PR at least. But we can continue to discuss here or in a separate issue the full build error. Or just not worry about it since in practice Circle is the only one who needs to suffer through full builds :)

[britta-wstnr]

Up to date pyvista and pyvistaqt?

Yup, fresh conda install (pyvista 0.42.3 and pyvistaqt 0.11.0)
But indeed, I don't run full builds - so fine for me to not look further into. I just found it puzzling!