Add document about Create a new cop

[ydah]

Follow up: https://github.com/rubocop/rubocop-rspec/pull/1890

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with main (if not - rebase it).
• [x] Squashed related commits together.
• [ ] Added tests.
• [ ] Updated documentation.
• [ ] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).