Add rubocop-rspec_rails to suggested extensions and extension doc

[ydah]

This PR adds rubocop-rspec_rails to the SuggestExtensions list and extension doc.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[aprescott]

This appears to have resulted in the following suggestion message:

The following RuboCop extension libraries are installed but not loaded in config:
  * rubocop-rspec_rails

I assume this is related to rubocop/rubocop-rspec#1830 which extracted RSpec Rails cops to their own gem.

From reading #9156 and #9151 (comment) I would have thought this suggestion message wouldn't appear? rubocop-rspec_rails is indeed a (transitive) dependency for the app generating above the suggestion, and #9156 seems to suggest that in this case no such message should appear:

    rubocop-rspec (2.29.1)
      rubocop (~> 1.40)
      rubocop-capybara (~> 2.17)
      rubocop-factory_bot (~> 2.22)
      rubocop-rspec_rails (~> 2.28)
    rubocop-rspec_rails (2.28.2)
      rubocop (~> 1.40)

I can of course fix the suggestion message, but I thought I'd mention it in case this is a sign of some other bug. The presence of the suggestion message implies there's missing config, which I don't think is the case, and addressing the extra suggestion message noise feels like unnecessary work given that there's no actual change happening yet.

[aprescott]

Maybe also worth calling out: rubocop-rspec's 2.28.0 changelog indicates the extraction is intended to be invisible:

the change will be invisible to users until then