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-performance cops.

Before

$ bundle exec rubocop --show-docs-url Performance/AncestorsInclude
https://docs.rubocop.org/rubocop/cops_performance.html#performanceancestorsinclude

After

$ bundle exec rubocop --show-docs-url Performance/AncestorsInclude
https://docs.rubocop.org/rubocop-performance/cops_performance.html#performanceancestorsinclude

---

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)