Boatload of header cleanup
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: