Make Charts release happen similar to omnibus-gitlab
Spinoff from #1337 (closed)
Currently, tagging and publishing of Charts happen in a single pipeline, in an uncontrolled manner (i.e, we don't get to test the tagged version of Charts before it gets published). This is less than ideal. With gitlab-org/build/CNG!235 (merged) and gitlab-org/release-tools!645 (merged), we will be making the release of CNG happen similar to that of omnibus-gitlab: Release will be done by a manual job which will be played by release-tools. Also, Charts pipeline will be getting a wait-for-images logic by !825 (merged).
Similarly, we should restructure the Charts release to follow a similar pattern - Make the job which calls GitLab Pages pipeline a manual one, and play it from release-tools.