Adds MR performance guidelines to left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!108606 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:
  • Already live at https://docs.gitlab.com/ee/development/merge_request_concepts/performance.html

Related links

• Related to Moves MR performance requirements page into subdir (gitlab!108606 - merged) which introduced the page
• Related to Create new "Merge Requests" development docs se... (gitlab#386620 - closed) where the work is being done