doc: use sentence-case for headings in docs

PR-URL: https://github.com/nodejs/node/pull/33889
Refs: https://developers.google.com/style/capitalization#capitalization-in-titles-and-headings
Refs: https://docs.microsoft.com/en-us/style-guide/capitalization


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