Add subscription troubleshooting page to nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!164357 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://lc-add-troubleshooting-subscription-nav.docs.gitlab-review.app/ee/subscriptions/gitlab_com/gitlab_subscription_troubleshooting.html