Use Title Case in h1 headings and sentence case in all others

- I wrote about the implications of (not) doing this in the PR
  (https://github.com/Homebrew/brew/pull/6826#issuecomment-564273209),
  but in summary:

  - It looks like pretty much the only place we've ever adhered to the
    sentence case rule is the Prose Guidelines itself.
  - Correcting all these to sentence case would be a lot of work (and
    sometimes not even make sense).
  - Admittedly, some of the title case ones look a bit horrible, but
    even with some allowed exceptions, but it's better than being
    consistently inconsistent.
  - Only lint for title case in h1 headings. H2s and H3s etc. can be
    sentence case. There's not currently a lint for that.