Merge branch 'docs/git-submodule-absolute-urls' into 'master'

Document support for absolute submodule URLs in GitLab Runner

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/117562



Merged-by: Fiona Neill <fneill@gitlab.com>
Approved-by: Arran Walker <ajwalker@gitlab.com>
Approved-by: Fiona Neill <fneill@gitlab.com>
Co-authored-by: Nejc Habjan <nejc.habjan@siemens.com>
Co-authored-by: Nejc Habjan <hab.nejc@gmail.com>