Update left nav to fix broken links

The internal_links job is failing: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/1877756637

Probably because of the recent links cleanup MRs:

• !2305 (merged)
• gitlab!76438 (merged) (The Services and Event tracking pages were removed in this MR)

This MR removes the Services API page from the left nav (this API has been renamed to 'Integrations') and changes the 'Event tracking' to point the handbook page, per the redirect.

• Give the MR a descriptive title.
• Link to the merge request that introduced the new page.