[4.x] Clean up Section fieldtype styles

[caseydwyer]

This PR cleans up the styles of the Section fieldtype, by addressing a few bits that were just slightly off:

• Unequal space on top & bottom of the label/description—loosely related to [4.x] Remove bottom margin on a Section field type publish form when no instructions are present #8371
• Corner radius was one step smaller (.rounded vs. rounded-md) than the ancestor .card.
• Pulled away from the bottom edge, due to top:1px; replaced with a negative margin-top.
• Immediate sections with a header get pulled up, breaking into the header's border.
• Fixes double-stacked borders, when sections are back to back.
• Bottom radius not accounted for, when the section is at the end of a card.

Those last two were mostly to give a better foundation for addons (eg, Group fieldtype) that use sections to "contain" other fields. But still cleaner to have in core, without adding too much into the stylesheet.

Here's a quick kitchen sink setup, to show the before-and-after. Tweaked a few other things in the code after this was rendered, but the most noticeable changes are shown here:

Lmk if you'd like to see any adjustments, and I'll take a crack at it. Thanks team!

[jackmcdade]

I love this, thank you! Will give it a double-check asap <3

[caseydwyer]

Great, thanks Jack (and no rush from my end)! 👍

[jackmcdade]

Looks great, I love it. Thanks again! ❤️

[caseydwyer]

No problem, thank you!