Add registry webhook notifications dashboard

The container registry emits metrics around its webhook notification system, however, there are no dashboards currently displaying this information. This MR adds a small dashboard with the current available metrics as described in the registry documentation.

Related to Expose webhook notification metrics in Grafana (gitlab-org/container-registry#828 - closed)

Sample screenshot

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Generated dashboard below

https://dashboards.gitlab.net/dashboard/snapshot/TNDzDE7jJ1Ra7ENm8IQD1pIBwDaZuQdP