Add check that page title is in sync with ToC, h1, and metadata

[Eric-Arellano]

You have to set the page title in three places:

• _toc.json
• H1, i.e. starting with #
• Metadata at the top of the page

It's actually quite useful that these three can be different values. However, ~90% of the time, we want them to be in sync. We should require allowlisting if they are not in sync.

Background for this idea: someone was confused why searching for the _toc.json title was not resulting in the page showing up.

allowlisting

TBD the best approach to do this. The allowlist will need to be the file name. But do you only list the file to say it's okay for them to diverge, or do you have to get more granular to say which of the 3 are divergent? (Imo the former is less confusing and better)