Split Pod concept into a simple introduction and further details

[shirady]

I think this page needs to be separated, today it includes basic information about pods as well as detailed advanced subjects.

https://k8s.io/docs/concepts/workloads/pods/

This page should include:

• What is a Pod?
• Using Pods: Workload resources for managing pods
• Working with Pods: Pod template
• any basic information

... all the rest are detailed advanced subjects, and if someone did not know what is a pod, he will hardly understand those details.