document the way to run control plane pods on infra node.
implementation done in https://issues.redhat.com/browse/SRVKP-1544

Notes from Siamak in the related Epic:

Goal
Add docs that explain how to run OpenShift Pipelines control plane (Tekton controllers, etc that run in openshift-pipelines namespace) on the infra nodes and customer considerations in collocating these controllers on the infra nodes (e.g. possible effect on other infra services).

Background
Customers are allowed to run the OpenShift Pipelines control plane in the infra nodes in order to reduce the cost of the platform given that customer don't have to pay for the compute used on infra nodes:
https://docs.google.com/document/d/1sk5vCeYoAVNd7oXBFpGaX8DUhzuqSdLDOuNa2ZusfQc/edit

Acceptance Criteria
Customers can configure (opt-in) OpenShift Pipelines control plane to run on infra nodes
OpenShift Pipelines upgrade maintains infra node configurations on upgrade
Docs exist for using infra nodes for OpenShift Pipelines control plane

Refer: https://docs.openshift.com/container-platform/4.7/architecture/control-plane.html#architecture-machine-config-pools_control-plane