Add new upgrade troubleshooting index page to global nav with submenu

What does this MR do and why?

For gitlab-org/gitlab#555861, and as a companion to gitlab-org/gitlab!200912 (merged), add new upgrade troubleshooting index page to global nav with submenu.

Preview: https://docs.gitlab.com/review-mr-1110/update/troubleshooting_and_rolling_back/

Merge request acceptance checklist

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• I have evaluated the MR acceptance checklist for this merge request.