Style: Fix vertical margins of def lists w/o <p>s to match those with

[CAM-Gerlach]

In #2300 , it was reported that the vertical margin varies substantially between entries with multiple paragraphs (i.e, another <p> element) and those that only have one (i.e. just text, no <p>). Furthermore, the margins are still pretty tight as @pradyunsg reported, making them uncomfortable to read; they should be at least equivalent to the normal margin between paragraph-level elements.

PEP 639 is a canonical example without and with this PR.

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This resolves both of those concerns by just ensuring that dl dd (except headers and footnotes, which are already overridden) have the same 0.5rem bottom vertical margin as paragraphs do:

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@pradyunsg Are there any additional vertical line spacing concerns you'd like me to address at this time?

Resolves #2300

[pradyunsg]

@pradyunsg Are there any additional vertical line spacing concerns you'd like me to address at this time?

Just one: The line-height set on html should probably change from 1.4rem to just plain 1.4. I don't think I've ever seen someone specify the unit in rem, for line-height. :)

[pradyunsg]

Actually, looking at PEP 101 again, I'm finding it a bit difficult to specifically point to what exactly looked off to me. I'll file a PR if I'm able to place what it is -- instead of bothering people about the other nebulous whitespace concerns I may have. :)

[CAM-Gerlach]

Seems like this is good to go for now, so I'll go ahead and merge it.