Fix margins of definition lists in the Sphinx theme

[AA-Turner]

... we really need to fix the margins of the definition lists in the Sphinx theme. There is almost no margin/padding between definitions, much less than even normal <p>s, making it very hard to read, while there is at least the normal (adequate) spacing if the definition has multiple paragraphs.

Originally posted by @CAM-Gerlach in #2298 (comment)

[AA-Turner]

I've a further implementation PR I'm working on that does the reference title changes -- I'll look into this whilst preparing that one.

A

[CAM-Gerlach]

@AA-Turner It looks like this is still outstanding, so if you're busy I can tackle this.

[CAM-Gerlach]

@pradyunsg mentioned on the styling thread that definition lists are also missing indentation on the definition on the definition, which can be fixed here too, and perhaps co-opting some other styling (vertical rules) from the previous system and other themes.

If you don't get to this in 24 hours, I'll go ahead and take care of it instead, since its just frontend CSS rather than any backend changes.