Add 3d backend table for overview

[GuillaumeFavelier]

As we discussed @larsoner, this PR adds two tables to have an overview of the capabilitiies of each 3d backend available.

As of now, only Mayavi and PyVista are available but work is in progress to add VisPy and Ipyvolume too.

The first table contains all the 3D features supported by the backend and show the level of progression towards a full integration of those features into mne-python.

The second table is a list of the 3D functions currently supported and tested for this backend.

Those tables will be updated as more features will be added and the integration of the backend will improve. As expected, each new backend will add a column to those tables and once enough features are available, the PR for the "candidate" backend could be merged eventually.

For now, the convention used is 'X' for full support, '-' for partial support and ' ' (space) for not supported.

Of course, any suggestion to improve or any feedback is welcome.

[codecov]

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[larsoner]

Pushed a cosmit to clean up the table render

[larsoner]

https://13335-1301584-gh.circle-artifacts.com/0/dev/generated/mne.viz.set_3d_backend.html#mne.viz.set_3d_backend

Thanks @GuillaumeFavelier !

[larsoner]

Feel free to rebase whichever backend you think is sufficiently mergeable and add to the table