Add CI Decomposition QA job
Related issues gitlab#353649 (closed) and https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1247
Follow-up on !918 (merged)
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 Nailia Iskhakova