Add monorepo troubleshooting to global navigation

What does this MR do and why?

For: gitlab!138397 (merged), add a global navigation item.

Preview: https://eread-add-monorepo-troubleshooting-to-global-nav.docs.gitlab-review.app/ee/user/project/repository/monorepos/troubleshooting.html.

CC @jcaigitlab

Merge request acceptance checklist

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