doc: standardize on sentence case for headers

Previously, our documentation headers were a mixture of title case,
sentence case, and things that were neither. For technical
documentation, the _de facto_ standard seems to be sentence case. (See
refs below.) So let's standardize on that. This commit follows a
commit implementing this standard. This commit adds it to the style
guide.

Refs: https://developers.google.com/style/capitalization#capitalization-in-titles-and-headings
Refs: https://docs.microsoft.com/en-us/style-guide/capitalization

PR-URL: https://github.com/nodejs/node/pull/33889


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>