docs: Add example to pod placement

[sbernauer]

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[lfrancke]

I'm sorry but: How does this look in a rendered version now?
I wanted to avoid the duplication of "Pod placement"
And: It should be "placement" not "Placement"

And: You have "Pod" once and "pod" another time. We should be consistent

[sbernauer]

Fixed lower and upper case.

Currently it reads

The concepts page on Pod placement describes how you can configure Pod placement in general.

We could change it to e.g.

This concepts page describes how you can configure Pod placement in general.

but I have been told to not have "click this" links, but instead tell the user where the link is pointing too.
But I don't have a strong opinion and let you decide

[lfrancke]

My suggestion:

For general configuration of Pod placement, see the [Pod placement concepts] page.

[sbernauer]

Applied in 2996c2f

[lfrancke]

Sorry, reading this again now, I have a new suggestion.

Suggested change

	1. Co-locate all the HBase Pods (weight 20)
	2. Co-locate HBase regionservers with the underlying HDFS datanodes (weight 50)
	3. Distribute all Pods within the same role (masters, regionservers, rest servers) (weight 70)
	1. Co-locate all the HBase Pods (weight 20)
	2. Co-locate HBase regionservers with the underlying HDFS datanodes (weight 50)
	3. Distribute all Pods within the same role across nodes so multiple instances don't end up on the same Kubernetes node (masters, regionservers, rest servers) (weight 70)

Point 1 needs clarification but I don't know exactly how. What is it we do exactly there?
And we should point out (I thought we did somewhere) that this is "only" preferred not required

[sbernauer]

See 30eba1a

[lfrancke]

Thank you. But what does the first point actually mean?
"Co-locate al the HBase Pods"

[sbernauer]

Its the first rule in the example below

affinity:
  podAffinity:
    preferredDuringSchedulingIgnoredDuringExecution:
    - podAffinityTerm:
        labelSelector:
          matchLabels:
            app.kubernetes.io/instance: cluster-name
            app.kubernetes.io/name: hbase
        topologyKey: kubernetes.io/hostname
      weight: 20
  podAntiAffinity:
    preferredDuringSchedulingIgnoredDuringExecution:
    - podAffinityTerm:
        labelSelector:
          matchLabels:
            app.kubernetes.io/component: master
            app.kubernetes.io/instance: cluster-name
            app.kubernetes.io/name: hbase
        topologyKey: kubernetes.io/hostname
      weight: 70

I just pulls all HBase Pods together, so that e.g. hbase master + regionserver + restserver run on the same node to reduce latency.

[sbernauer]

Thanks!