Improve spacing in paragraphs

Problem/Motivation

In paragraph sections, we can set padding-top and padding-bottom. Because of this, the first inner element of a paragraph should have margin-top: 0. Currently, this is not the case.

For example, if you add a Sidebar embed paragraph with a title (but no icon), the title gets a margin: var(--heading-margin);, which causes inconsistent spacing — the top spacing ends up larger than the bottom.