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.

docs/Prose-Style-Guidelines.md

@@ -41,7 +41,7 @@ We prefer:
 
 ### Structure and markup
 
-* Sentence case in section headings, not Title Case
+* Title Case in `h1` headings; sentence case in all other headings
 * Periods at the ends of list items where most items in that list are complete sentences
 * More generally, parallel list item structure
 * Capitalise all list items if you want, even if they're not complete sentences; just be consistent within each list, and preferably, throughout the whole page

docs/vale-styles/Homebrew/Titles.yml

@@ -1,6 +1,10 @@
----
 extends: capitalization
-message: "'%s' should be in sentence case"
+message: "'%s' should be in title case"
 level: warning
-scope: heading
-match: $sentence
+scope: heading.h1
+match: $title
+style: AP
+exceptions:
+  - brew(1)
+  - Homebrew/homebrew-core
+  - Homebrew/linuxbrew-core