Add docs on configuring RSpec aliases from gems

[pirj]

Better viewed as https://github.com/rubocop-hq/rubocop-rspec/blob/add-doc-how-to-configure-rspec-aliases-in-third-party-gem/docs/modules/ROOT/pages/third_party_rspec_syntax_extensions.adoc

fixes #1077

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Before submitting the PR make sure the following are checked:

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

[Darhazer]

Thank you 🙇

[bquorning]

I think we pretty consistently use either lowercase rubocop-rspec with a dash, or titlecase RuboCop RSpec with a space.

[pirj]

I think we pretty consistently use either lowercase rubocop-rspec with a dash, or titlecase RuboCop RSpec with a space.

Indeed.
I've added a commit to remove remaining mentions of "RuboCop-RSpec" with a dash. I've copied one of it over which is a perfect example of how errors proliferate.