Add support for configuring additional network policies

[rxbn]

This PR adds support for configuring additional network policies for Prometheus, Alertmanager and Grafana.

Checklist

• [x] The PR has a meaningful title. It will be used to auto generate the changelog. The PR has a meaningful description that sums up the change. It will be linked in the changelog.
• [x] PR contains a single logical change (to build a better changelog).
• [x] Update the documentation.
• [x] Categorize the PR by adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog.
• [x] Link this PR to related issues or PRs.

[rxbn]

Documentation added, I am going to merge this for now, further refinements can be added later on.