Added opening slash to relative links for handbook
I noticed there were some 404s in the handbook caused by relative links missing an opening /. Updated to fix those 404s.
The 17.0 major release is coming on May 16, 2024! This version brings many exciting improvements to GitLab, but also removes some deprecated features. We are introducing three breaking change windows during which we expect breaking changes to be deployed to GitLab.com. You can read more about it on our blogpost. The first breaking change window begins 2024-04-22 09:00UTC and ends 2024-04-24 22:00UTC.
I noticed there were some 404s in the handbook caused by relative links missing an opening /. Updated to fix those 404s.