Add frontend diffs guide page to left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!109229 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Related issues and merge requests

• Related to Create documentation for frontend diffs Vue app (gitlab!109229 - merged) which added the page
• Related to Add frontend details to Merge Requests diffs te... (gitlab#388141 - closed)
• Related to Convert excalidraw flowchart to mermaid (gitlab#388843 - closed) work that remains on this page