Add CI check to compare docs to system tables

[Blargian]

Motivation

To prevent sections of the docs getting removed accidentally such as what happened in #2948.
Or #2724 where auto-generated setting script failed and settings page was empty.

Feature

An idea to implement some form of basic regression testing with Github actions to check docs content against system tables, and contribute to the goal of having documentation be complete.

As an idea for something rudimentary we might pull all top level (# header1) tags on some page and compare them to the system tables to check there is a corresponding docs entry for some feature. For instance pulling all top level tags on arithmetic-functions page can be used to generate a list of the documented functions. We can compare it to system.functions to ensure that we have a section of the docs corresponding to each function in the system table. If a sections of the docs get accidentally removed then the CI check will fail on the now missing entry.

This can also be extended in the future for more specific checks. eg. for checking that feature tiers (beta, experimental, production, obsolete) in the docs are up to date with those in the system tables. If a feature changes from beta to production in the system table but is not updated in the docs then the CI check will fail.