Make wide images and tables horizontally scrollable

[mgeier]

See #61.

This avoids horizontal scrollbars around the whole text.

Rendered:

• https://insipid-sphinx-theme--66.org.readthedocs.build/en/66/showcase/images.html#scaled-image
• https://insipid-sphinx-theme--66.org.readthedocs.build/en/66/showcase/tables.html#large-table

[mgeier]

Turns out that this doesn't work for right-aligned images (and floating images in general).

I've created an alternative in #68.