Add v2 update docs

[pirj]

Better viewed as https://github.com/rubocop-hq/rubocop-rspec/blob/add-v2-migration-docs/docs/modules/ROOT/pages/upgrade_to_version_2.adoc

TODO

• adjust docs related to the changes from Remove relevance detection code #1063
  • that Include should be used instead of Patterns
  • note that Include's merge mode is set to override by default
• adjust docs related to the final changes to API from RSpec aliases runtime matchers #956
• • syntax changes
• • mention the change of ExampleGroups::GROUPS to ExampleGroups#regular and from Examples::EXAMPLES to Examples#regular https://github.com/rubocop-hq/rubocop-rspec/pull/956/files#r516238107
• • # call super if you define on_new_investigation in a cop
• • do not use let(:cop) { described_class.new } in custom cops' specs, as this will break specs (config not loaded)
• • add a section on how to configure DSL aliases

---

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).

[pirj]

@sl4vr appreciate if you could take a look.

[Darhazer]

Great job! 🙇

[pirj]

Extended the guide in regards to this change rubocop/rubocop#8490

[pirj]

@bquorning Green.
Besides all, this includes information on how to migrate to 2.0 given we decide to merge #956 in its current form.

[bquorning]

Sorry about the many nitpick comments here.

I believe this PR should be the last one to be merged into release-2.0, as it may still change depending on the other PRs getting merged.

[pirj]

Thanks for proofreading, @bquorning !

I believe this PR should be the last one to be merged into release-2.0

Agree 👍

[pirj]

All review notes applied ✔️

[pirj]

@bquorning Ready for the review. There are changes since the last review, the list is in TODO section of the ticket description.
I've rearranged all the commits.

[sl4vr]

Just found a few inaccuracies.

[pirj]

Eagle eye, @sl4vr, thanks a lot!

[bquorning]

I don’t understand this sentence. Could you explain a bit more, please?

[pirj]

Suggestions applied, clarifications made, fixups pushed, later squashed. Thanks for the review, @bquorning !

[pirj]

edge-rubocop is back to green! 💚