This commit includes some general improvements to heading styles
(font-weight, margin/padding). It also adds a horizontal separator above
`<h2>` headings to segment important sections more easily.
(As a consequence, we can remove several `<hr />` elements right before
`<h2>` headings.)
Co-authored-by: Stefanie Fluin <sjtrimble@gmail.com>
PR Close#40427