Improve volumes concept page

[network-charles]

Description

This PR improves the volumes page by adding an introduction to what a volume is. It also improves the section title so it's comprehensible for beginners.

Issue

This PR addresses the below comment in issue #31159.

For beginners with little knowledge of k8, the Volumes should just give a very general picture of managing storage, instead the concept page gives a long list of different type of volumes which most beginners don't really need at this point.

[k8s-ci-robot]

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[sftim]

How about this?