Move gitlab_rails_cheat_sheet - "Authentication" to relevant page

Link to specific heading: https://docs.gitlab.com/ee/administration/troubleshooting/gitlab_rails_cheat_sheet.html#authentication

The listed section from our troubleshooting guide needs to be reviewed and moved.

1. Do a technical accuracy review.
   • If the information is a duplicate or is to fix a bug that's no longer reproducible, consider whether it's still required. If we no longer need it, make a MR to remove the content instead.
2. If the content covers a rail command, look to see if there is an open issue in the snippets project that covers the same thing. Decide which version to keep, and close the snippets issue with a link to your docs issue.
3. Find the most appropriate (feature) page outside of the troubleshooting section of our docs to move the troubleshooting section to if one is not suggested above.
4. Double check if the information is duplicated. If partially duplicated, integrate the troubleshooting section to the content on feature page and ignore the next 2 steps.
5. If a troubleshooting section does not already exist at the bottom of the feature page, uncomment the heading (if present) or create it ## Troubleshooting [feature].
6. Move the content to the troubleshooting section on the feature page.
7. Ensure the heading for your content includes "in Rails console" and the first line after the header includes a link to how to [start a rails console session](../administration/operations/rails_console.md#starting-a-rails-console-session).
8. Ensure the content follows the the troubleshooting content/structure style. If you have style guide questions, feel free to ask in the #docs Slack channel or submit the MR and make comments for the Technical Writer to review.
9. Create a MR with the changes and follow the usual process for review/assignment.