docs: update usage of rubocop-rails integration

[tnir]

Per updating the native support of engines and Packwerk in rubocop/rubocop-rails#1476, users can use rubocop-rails out of the box, in general. They might need to tweak their own RuboCop config for their own custom Cops.

[tnir]

@alexevanczuk What do you think?

[dduugg]

Thanks for the docs update — the old instructions were misleading for anyone on a modern version of rubocop-rails, so this is a welcome improvement.

A few things worth addressing before merging:

Grammar fix (required):
"If you has your own custom cop" → "If you have your own custom cop"

Suggestion — add context for the version requirement:
The new text says "Use rubocop-rails 2.32.0 or higher" but doesn't explain why. A brief note would help readers understand without having to follow the linked PR. For example:

Version 2.32.0+ includes native support for pack paths, so built-in cops no longer require manual Include overrides.

Suggestion — clarify the example cop names:
Custom/CustomCopOne and Custom/CustomCopTwo are reasonable placeholders, but the Custom namespace may confuse readers. Consider using something like YourOrg/CustomCop or adding a comment like # Replace with your custom cop name to make the intent clearer.