Add CI Decomposition QA job
Related issues https://gitlab.com/gitlab-org/gitlab/-/issues/353649 and https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1247
Follow-up on https://gitlab.com/gitlab-org/gitlab-qa/-/merge_requests/918
The MR adds a new QA job to test GitLab with a decomposed DB. Also it adds docs how to run ci_decomposition
config with GitLab QA.
Edited by George Koltsov