Improve documentation for controller manager metrics

## Problem

When users try to monitor their Kubernetes application, they don't have a way to understand our predefined dashboards without having solid knowledge about Kubernetes.

Right now, the documentation for that can be found in [this page](https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-module-kubernetes.html). However, this is not enough to understand all the metrics and the meaning of each one in [here](https://www.elastic.co/guide/en/beats/metricbeat/current/exported-fields-kubernetes.html#_controllermanager) doesn't it make it clearer.

## Solution

Create a new documentation that **highlights the metrics available on the controller manager dataset and which problems we can detect with those**. With this new document we can expect to have the answer for these type of questions:

- What can we conclude from the unfinished work of some workqueue? What does that mean?
- Why is it important to track the CPU time spent?
- What is the take away on resident memory variations?

This new page would use visualizations to make the answer for these type of questions more clear. This page **should not use the predefined dashboard**, as a change on it would require an update to this page.