DOC: Add version switcher

[scott-huberty]

Since we are in a period of active development, it's probably a good time to add a version switcher to the doc website.

For reference, MNE-BIDS sets it up in their doc.conf file:

https://github.com/mne-tools/mne-bids/blob/b8b42110a5de52c731b0cc35cc9610e55abdc255/doc/conf.py#L157-L161

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