Document dashboards and alerts offered by the mixin

[pracucci]

What this PR does:
In this PR I'm proposing to add a couple of documentation pages to give some instructions about our dashboards and alerts.

I tried to do my best to follow these guidelines but since it's the first time writing doc based on that I may have done many mistakes.

Out of the scope of this PR:

• Extract dashboard description from jsonnet sources
• Add dashboard screenshots to the list of dashboards (I will try to automate the screenshot taking in a separate PR)

Which issue(s) this PR fixes:
Fixes #909

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[osg-grafana]

@chri2547, I would like to pair with you on this PR as a first go at aligning with our new refactored information architecture. @pracucci, this likely means that I will create a new PR and attempt to merge it into this one.

[pracucci]

@pracucci, this likely means that I will create a new PR and attempt to merge it into this one.

I'm not sure to understand why you need to re-create this PR. To me would be more beneficial if you provide me feedback, so I can learn as well and apply learning in upcoming other documentation improvements.

[osg-grafana]

@pracucci, this likely means that I will create a new PR and attempt to merge it into this one.

I'm not sure to understand why you need to re-create this PR. To me would be more beneficial if you provide me feedback, so I can learn as well and apply learning in upcoming other documentation improvements.

I intend to provide rationale for the changes to provide guidance about how we are thinking about information architecture, which is also quite new for us writers. I thought that a new PR might be the easiest way to accomplish that.

[osg-grafana]

User error, need to reopen.

[osg-grafana]

@pracucci, this likely means that I will create a new PR and attempt to merge it into this one.

I'm not sure to understand why you need to re-create this PR. To me would be more beneficial if you provide me feedback, so I can learn as well and apply learning in upcoming other documentation improvements.

I intend to provide rationale for the changes to provide guidance about how we are thinking about information architecture, which is also quite new for us writers. I thought that a new PR might be the easiest way to accomplish that.

Will commit directly to this branch per your suggestion on Slack. In doing so, I will provide rationale for each change.

[osg-grafana]

Approving so that I am not blocking folks.

[osg-grafana]

@jdbaldry, we can take a subsequent look at this area of the docs in light of #732 when we have a chance to cover this area.