Reduce padding in some container elements

[mortenpi]

Reduces and makes the padding more consistent between various container elements. For admonitions, it changes as follows:

It also makes the padding in the docstrings consistent with the admonitions, although I left the content vertical padding a little higher, since docstrings often start with a code block, and 0.5rem looked a little too narrow then:

For code blocks, the vertical padding is slightly reduced, but left padding actually increases (to be consistent with admonitions and docstrings):

@KristofferC: what do you think? Not quite as narrow as you suggested, but perhaps a good compromise? Close #1814.

[KristofferC]

Yes, looks a lot better to my eyes. Thanks!