Follow Jsonnet monitoring-mixin structure

[maxbrunet]

It would be nice if the Jsonnet dashboards could follow the mixin structure described here: https://monitoring.mixins.dev/

This would require to make the dashboards and their dependencies (e.g. grafonnet) easy to install with jsonnet-bundler and available under a grafanaDashboards object where keys are filenames and values their content.

I can make a PR to restructure, and afterward, suggest some alerts to complete it.

Also related #47