Refactor a dedicated RuboCop development guide

What does this MR do?

This refactors existing content to make a dedicated development guide for RuboCop.

It's come out of a personal experience of not being able to find this information easily in the past 🙂.

The changes:

• Move what was previously titled as a "Ruby, Rails, RSpec" style guide but contained only information on how to implement a RuboCop rule to a dedicated page. This helps make the content more discoverable.
• Make the existing Ruby Style Guide page a better SSOT for Ruby style guide information. Clarify that our style guide is based on RuboCop rules, certain external style guides, and what is documented in the guide.
• Remove the recommendation to document RuboCop styles in the Ruby style guide, and instead recommends documenting styles that we don't back with a RuboCop rule, or that we have no opinion about (also see Slack thread internal, good for 90 days).

Closes #375605 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.