docs: reorganize rulesets and custom rulesets pages

[heitortsergent]

Chore summary Take a pass at reorganizing the rulesets and custom rulesets into its own section, and breakdown information into separate pages. The custom ruleset page specifically has a lot of information in it, and I think it would be helpful if we could break it down into separate pages to improve navigation, and searching, and make it easier to expand specific topics.

One example of a topic that could have its own page is Overrides. And a user question that we could keep in mind when updating the topic.