PMM-12960 reworking k8s dashboards and polishing them

[spron-in]

https://perconadev.atlassian.net/browse/PMM-12960
Reworking k8s dashboards.

Remove Kubernetes Volumes
Reasoning: good dashboard, but not clear how to use it. We have volumes information in Databases on Kubernetes dashaboard as well.

Remove Pod Status
Reasoning: also good, but fails to provide insights if the Pod does not have limits set. Also too excessive, we have all this info in Databases on Kubernetes.

Remove Kubernetes Cluster Summary
Reasoning: it just duplicates Kubernetes Overview dashboard.

Rework Kubernetes Overview
Main changes:

1. Rename it to Kubernetes Cluster Overview
2. Do not allow to choose "all" clusters as it does not make sense.
3. Reorganize the tiles at the top, now they look neat and provide good level of details and structure
4. Polish the dashboard so it works with 2 or more clusters properly and also adheres to PMM standards

Rework Kubernetes DB Cluster
Main changes:

1. Rename to Databases on Kubernetes - we might want to expand it to other operators
2. Variables are now dynamic and smart. For example, it will not show you the namespaces where you don't have databases controlled by the Operator
3. Remove POD picker. We show information for the cluster
4. Provide information about the database and proxy in a separate rows
5. Show the limits for CPU and RAM if they are set - allows to quickly understand where utilization is
6. Add menus to quickly navigate to another dashboards for more database insights or get help in docs on from Percona
7. Polish the dashboard so it works with 2 or more clusters properly and also adheres to PMM standards