Add "Create Merge Request" to Web IDE docs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/120725



Merged-by: Lorena Ciutacu <lciutacu@gitlab.com>
Approved-by: David O'Regan <doregan@gitlab.com>
Approved-by: Lorena Ciutacu <lciutacu@gitlab.com>
Reviewed-by: Lorena Ciutacu <lciutacu@gitlab.com>
Co-authored-by: Michael Friedrich <mfriedrich@gitlab.com>