[Reporting] Document how to make screenshot reports work in ECK

[tsullivan]

Kibana version: 8.1

Original install method (e.g. download page, yum, from source, etc.): ECK

Describe the bug:
Under the main documentation for deploying Kibana in ECK, a Kibana container will not have optimal resources for running screenshot report jobs.

The following custom yaml for the Kubernetes Kibana instance optimizes the amount of system RAM to help PNG/PDF reports complete faster:

apiVersion: kibana.k8s.elastic.co/v1
kind: Kibana
metadata:
  name: reporting-custom-quickstart
spec:
  version: 8.1.3
  count: 1
  elasticsearchRef:
    name: quickstart # 1
  podTemplate:
    spec:
      containers:
      - name: kibana
        env:
          - name: NODE_OPTIONS
            value: "--max-old-space-size=800" # 2
        resources:
          limits:
            memory: 2Gi # 3
            cpu: 2

1. Refers to https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-deploy-elasticsearch.html#k8s-deploy-elasticsearch
2. Between 800 and 1200 (megabytes) should be enough for the Kibana process. Primarily, containers need system memory available for the memory-hungry Chromium process that is spawned.
3. The more memory here, the better. Depending on search latency, and the number of browser connections needed to load the PDF dashboard. Up to 8G could be necessary. 16G would put it at a "comfortable" level.

Refer to https://kubernetes.io/docs/concepts/configuration/manage-resources-containers/ for explanation of this configuration

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesUx)

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)