Improve Persistent Volumes concept page

**This is a Feature Request**

**What would you like to be added**

Make [Persistent Volumes](https://kubernetes.io/docs/concepts/storage/persistent-volumes/) easier for people to understand.

Especially, we need to explain the confusing idea of an “_ephemeral PersistentVolume_” - which in Kubernetes is absolutely a thing. Maybe there should have been an EphemeralVolume API, but there isn't and now we're stuck with it!

**Why is this needed**

We want people to learn how to design, manage and use Kubernetes storage. The existing pages are OK but could certainly be better.

**Comments**
/sig storage
/kind feature

Start with the Persistent Volumes concept page at kubernetes.io/docs/concepts/storage/persistent-volumes/ and read the surrounding storage documentation for terminology and context. Clarify how “ephemeral PersistentVolume” fits into Kubernetes storage, with the page becoming easier to understand for people designing, managing, and using storage.

Written by the indexing model from the issue text.