Update trainee backend maintainer template to include CODEOWNERS

Add an action item for updating CODEOWNERS

Also tweak the links accordingly, linking to the team data and move all the guides to the top action items.

Closes #3576 (closed)

Preview:

Basic setup

1. Read the code review page in the handbook and the code review guidelines.
2. Understand how to become a maintainer and add yourself as a trainee maintainer on the team page.
3. Assign yourself and your buddy to this issue, and set up a call / Slack conversation between the two of you.

Working towards becoming a maintainer

There is no checklist here, only guidelines. Remember that there is no specific timeline on this.

Ensure that you are getting enough MRs to review, and of varied types. It is helpful to add a discussion to this issue for each MR there, for any conversations you and your buddy want to have about the review itself, without cluttering the MR.

When you're ready to make it official

1. Create a merge request for team page proposing yourself as a maintainer.
2. Create a merge request for CODEOWNERS or CODEOWNERS.disabled, adding yourself accordingly, and ask your buddy to review it.
3. Ask your buddy to add you to gitlab-org/maintainers/rails-backend.
4. Keep reviewing, start merging 🤘

/label ~"trainee maintainer"