Document Installing and Using VPA in BigBang and Vertically Scaling Log Forwarder Pods

Since log forwarder pods like fluentbit & promtail are designed to have 1 pod per node via a ReplicaSet instantiation they are unable to be Horizontally Scaled when reaching their resource limits. They can still be Vertically scaled once VPA is installed.

• Add a document in docs/guides/using-bigbang/ for installing and configuring VPA to vertically scale fluentbit & promtail pods in the event they need more resources applied and there is headroom available.