MNT: Revert to mayavi as 3d backend for building the documentation

[GuillaumeFavelier]

This PR follows #7228 (comment) and reverts Circle's config to use mayavi to build the documentation until #7228 is fixed at least.

[GuillaumeFavelier]

time_viewer=True needs to be reverted in plot_visualize_stc.py

...
generating gallery for auto_tutorials/source-modeling... [ 83%] plot_object_source_estimate.py
generating gallery for auto_tutorials/source-modeling... [ 91%] plot_source_alignment.py
generating gallery for auto_tutorials/source-modeling... [100%] plot_visualize_stc.py
Makefile:44: recipe for target 'html_dev' failed
make: *** [html_dev] Terminated

Too long with no output (exceeded 10m0s): context deadline exceeded

Reference: https://circleci.com/gh/mne-tools/mne-python/18369?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

[codecov]

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[agramfort]

time_viewer viewer auto to keep the feature when we move back to pyvista hopefully before the release in 2 weeks...

[GuillaumeFavelier]

To get it back, it mostly depends on:

• BUG: Bug with multi-view screen capture #7228
• WIP: Use _Brain in plot_vector_source_estimates #7060
• WIP: Set time_viewer and show_traces to auto #7380

[agramfort]

to me #7380 is the blocker. I can live with the two others as WIP

…

[GuillaumeFavelier]

But bringing time_viewer='auto' is not compatible with letting mayavi as default 3d backend for documentation because of #7398 (comment)