Boatload of header cleanup

(Note: This is based on top of !13 (merged) and !8 (merged). If those are merged first, then we should rebase to make this look a little smaller.)

This generally sanitizes the headings/sections used in various pages, e.g.

• Several pages have sections which serve the same purpose but are labeled differently for no useful reason.
• We should use more consistent grammatical structure across headings (e.g. all headings look like "Verb the noun", notwithstanding the addenda and admonitions)
• The style guide indicates that headings should use sentence-case.
• Many titles had unnecessary words.

Note that this MR does not try to make any significant changes to the section-bodies, and it doesn't remove/replace sections.

For a summary of what sections are on each of the pages, see:

https://docs.google.com/spreadsheets/d/10J2wmHyd1zSgu8Nx09jaYBewjt32eAWNcG-5WbHWgMQ/edit?usp=sharing

ping @MikeyMJCO @seamuslee