Add Rubocop to project
Most, if not all, GitLab Ruby projects employ Rubocop: https://github.com/rubocop-hq/rubocop. gitlab-docs
should be like other GitLab Ruby projects.
This MR adds the necessary libraries and configuration (from https://gitlab.com/gitlab-org/gitlab-styles), but adds no automated checks or tests. This project won't pass a check as currently configured, so some code updates or rule tweaks are required.
Rubocop can be integrated into editors, or run at the command line, and via bundle
: https://gitlab.com/gitlab-org/gitlab/-/blob/87864b3047c23b4308f59c27a3757045944af447/lefthook.yml#L31.
CC @gl-docsteam for visibility.