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
This commit breaks up the testing document into nine total documents, with each document focusing on an existing section of the current testing documentation. There are no content changes.
PR Close#37957
