add alt text so images aren't duplicated in html

[ianhi]

Fixes #2908 by using the workaround of adding a unique alt text to every image as proposed spatialaudio/nbsphinx#162

I think that the stable docs are probably based off the 7.x branch? so i will make a backport PR as well.

cc @jtpio

[jtpio]

Nice, thanks @ianhi!

It would be great if we could enable the readthedocs preview, so we could check the rendered docs directly in the browser: #2921

[ianhi]

@jasongrout if you have a chance would you consider merging this + the sibling PR #3014? I've recently found myself recommending people avoid that docs page in favor of the raw notebook https://gitter.im/jupyterlab/jupyterlab?at=5fd2a46d2a954a51a9c34191 and this is the fix for that.