Sentence-case titles, remove gerunds

[hey-august]

See this reference: https://developers.google.com/style/headings

[samrrichards]

Just saw your request for help in the Write the Docs Slack - I'd be happy to help with this over the course of this week!

[samrrichards]

@hey-august I've started work on this and wanted to check in to see how many PRs you'd like to correspond to this issue. There's a pretty large number of files that should be updated to conform to the desired formatting, so it might make sense to break the changes up into multiple PRs, to make it easier to review.

If we go that direction, is there any upper bound on number of files or top-level sections per PR that you think would be most ergonomic? I was thinking maybe 2-3 top-level sections per PR, although we could go higher if that would make more sense.