Move and rename Code Owners page

What does this MR do?

As part of several rounds of recent work, more information had to be stuffed into the existing Code Owners page. (https://docs.gitlab.com/ee/user/project/code_owners.html) This page was already unwieldy and in need of revision / carving apart, but the addition of two new major H2s seals the deal.

As this page now needs to be broken apart into a parent page and child pages, step 1 is to create a subdirectory of this page. doc/user/project/code_owners.md becomes doc/user/project/codeowners/index.md. That gives us room for brief, easily understood URLs.

This MR contains commits that:

• Create the new subdirectory.
• Rename code_owners.md to codeowners/index.md
• Update the directory traversal in outbound links and images
• Update the location of inbound links pointing to this page

This MR does NOT split the pages apart and refine them into better pages.

Needed follow-ups:

• Update left nav: Update URL to Code Owners page (gitlab-docs!3744 - merged)
• Update links in www-gitlab-com: Update links to moved Code Owners docs (gitlab-com/www-gitlab-com!122667 - merged)

Related issues

• Related to Technical design doc for Code Owners (#219916 - closed)
• Related to Add code owner documentation details to explain... (#396315 - 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 you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• 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.