Add dedicated sandbox troubleshooting and upgrade procedures

For issue gitlab-com/gl-infra/delivery#19557 (closed)

This section of the document should answer "How do we start a sandbox and run an upgrade?" question.

Thanks to @dat.tang.gitlab for helping summarize the onboarding scenarios from various troubleshooting comment threads on the onboarding issue, and @nolith for the great demo.

The section on "Interaction between Instrumentor and GET during GitLab version upgrades" has been started, but it should be supplemented with more details (examples, files, etc.), so I put a "WIP" on it for now.

Added some formatting fixes to make this doc cleaner.