Fix incorrect documentation URLs when using rubocop --show-docs-url

[r7kamura]

Added DocumentationBaseURL for rubocop --show-docs-url [COP1,COP2,...] command to work correctly for rubocop-minitest cops.

Before

$ bundle exec rubocop --show-docs-url Minitest/AssertEmpty
https://docs.rubocop.org/rubocop/cops_minitest.html#minitestassertempty

After

$ bundle exec rubocop --show-docs-url Minitest/AssertEmpty
https://docs.rubocop.org/rubocop-minitest/cops_minitest.html#minitestassertempty

---

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.

[koic]

Is the following issue resolved?
rubocop/rubocop-rails#756 (comment)

[koic]

That's just my two cents.

Suggested change

	Include:
	- '**/test/**/*'
	- '**/*_test.rb'
	DocumentationBaseURL: https://docs.rubocop.org/rubocop-minitest
	DocumentationBaseURL: https://docs.rubocop.org/rubocop-minitest
	Include:
	- '**/test/**/*'
	- '**/*_test.rb'

[koic]

@r7kamura Ping :-)

[koic]

Thanks!