Adds BE graphql for DAG

What does this MR do?

• Adds StageType connection to PipelineType
• Adds StageType, GroupType and JobType and corresponding fields
• Adds two new policies - stage and group
• Adds docs

Screenshots

query 
{
 project(fullPath: "group/project") {
    pipeline(iid: 1) {
      stages {
        edges {
          node {
            name
            groups {
              nodes {
                jobs {
                  nodes {
                    name
                    
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• Merge request performance guidelines
• Style guides

#223263 (closed)