Create dedicated page for rails console

What does this MR do?

'Run this on the rails console' is used extensively in our docs.

There needs to be single place where 'starting a rails console' is documented, as the procedure for Omnibus, Source, Kubernetes and Docker all different slightly.

This was attempted before. However

• An audit of the troubleshooting docs identified a second single source of truth!
• Five months on, it's clearer to me that our docs refer to the rails console a fair amount.
• Its own page, with an entry in the side menu, makes it findable in its own right.

step 1

This MR starts by proposing a dedicated page within Administration/Operations.

next steps in this MR

If this looks ok, I'll then

• Add it to the menus in gitlab-docs in a dependent MR.
• changing doc locations
• Remove dupe content from administration/troubleshooting/debug.md and administration/troubleshooting/navigating_gitlab_via_rails_console.md
• Find and fix all the broken links in docs.

For this iteration, the kubernetes content will stay where it is, and the docker content doesn't, I think, exist anywhere yet.

Related issues

#228961 (closed)

Author's checklist (required)

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