Merge branch 'docs-workflow-rules-variables' into 'master'

Add note about predefined variables with workflow

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/136955



Merged-by: Fiona Neill <fneill@gitlab.com>
Approved-by: Axel von Bertoldi <avonbertoldi@gitlab.com>
Approved-by: Fiona Neill <fneill@gitlab.com>
Reviewed-by: Furkan Ayhan <furkanayhn@gmail.com>
Co-authored-by: Marcel Amirault <mamirault@gitlab.com>