[Improvement]Documentation-Concepts

[shanshanying]

We need a concept page to introduce terms in Kubeblocks, e.g. K8S concepts https://kubernetes.io/docs/concepts/

The Concepts part helps you

• learn about the KubeBlocks system, and
• learn about the abstractions KubeBlocks uses, and
• helps you to get a deeper understanding of how Kubeblocks works.

KubeBlocks Architecture

ClusterDefinition

ComponentDef

ClusterVersion

Cluster

Component

Addons

TBC