doc: more documentation about Rules

[CatHood0]

Description

I added some more documentation of what the Rules are within the editor and how they work (according to what I could understand). In addition to providing an example of a custom rule for anyone who wishes to have an example of a Rule that is not the default in the package.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the package version in pubspec.yaml files.
• All existing and new tests are passing.
• I have run the commands in ./scripts/before_push.sh and it all passed successfully

Breaking Change

Does your PR require developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.