Document Storage Requirements for KKP and KubeOne

As a user, I want to know what type of storage fulfils the requirements for Kubermatic Platform and Kubernetes clusters so that I can test my storage backend and optimize it.

Acceptante Criteria
* summarize the content of the etcd documentation: https://etcd.io/docs/v3.5/
* Outline the most important items at the beginning
* Provide some high quality links for this topic
* Use the content in KKP and KubeOne documentation

Start with the linked etcd v3.5 documentation, then review the existing KKP and KubeOne documentation for storage requirements. Summarize the key requirements with the most important items first and include high-quality links; the work is done when both documentation areas use the resulting guidance.

Written by the indexing model from the issue text.