docs: Update new rules documentation

[barbeau]

Summary:

This PR attempts to simply the new rules documentation by taking the approach of providing an example, and then walking through sections of the example for both FileValidators and SingleEntityValidators, but always presenting FileValidators file to follow the initially introduced example of the TripUsabilityValidator. It also attempts to shorten text when possible and provide some additional context when needed.

It also fixes references to the SingleEntityValidator sections, some of which mixed references to FeedExpirationDateValidator and FeedServiceDateValidator.

Expected behavior:

New rules documentation should present an example and then walk the reader through it, providing only the basic information needed to understand the example.

A lot of text in-line was changed with existing text moved around, so it will probably be easier to view this change in it's entirely by clicking on the 3 dots in the "Files changed" tab and doing "View file".

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[lionel-nj]

Thanks @barbeau for this more narrative version! Just a few minor changes in line.

[barbeau]

Thanks @lionel-nj! I committed your suggestions, and fixed another typo I found.

[lionel-nj]

👍🏾 lgtm!