DOC: Main figure is small in recent builds

[effigies]

The main figure is now small. Main other change is upgrading Sphinx 4.5 to 5.0.

Investigation and PRs welcome!

1.8.2

https://nipype.readthedocs.io/en/1.8.2/

1.8.1

https://nipype.readthedocs.io/en/1.8.1/