fix: ci variables links are broken

Overview

This MR fixes the gitlab.com links in descriptions of CI variable being broken.

The fix is implemented by replacing all relative URLs with absolute URLs to the gitlab.com base path of the predefined variables (doc/ci/variables/predefined_variables.md).

Screenshot

Before	After
Screen_Recording_2021-04-01_at_22.30.21	Screen_Recording_2021-04-01_at_22.27.56