add configure doc for empty_example_group.rb

[kuboon]

based on v1 doc https://www.rubydoc.info/gems/rubocop-rspec/1.15.0/RuboCop/Cop/RSpec/EmptyExampleGroup

and upgrade doc. https://github.com/rubocop/rubocop-rspec/blob/cd6a30efa9094e389cf9e4dd473b7631fe18bab1/docs/modules/ROOT/pages/upgrade_to_version_2.adoc#adjust-the-configuration-of-rspecemptyexamplegroup

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [ ] Added tests.
• [x] 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).

[kuboon]

https://github.com/rubocop/rubocop-rspec/blob/v2.22.0/lib/rubocop/cop/rspec/multiple_expectations.rb#L51-L62

https://github.com/search?q=repo%3Arubocop%2Frubocop-rspec%20%40example%20configuration&type=code