Add helm-docs to auto-generate README
Implementation of #46 (closed)
This MR add a CI action that checks if README.md is in sync with values.yaml. We still expect the contributor to update the README themselves locally.
Example runs:
- no differences detected (ok): https://gitlab.com/schurzi/gitlab-agent/-/jobs/5770204293
- files our of sync (error): https://gitlab.com/schurzi/gitlab-agent/-/jobs/5770196630