Revise https://etcd.io/docs/v3.5/tutorials/how-to-setup-cluster/

**This one is probably a little more complex!**

This issue was created in preparation for the Contributor Summit docs sprint.

File: https://github.com/etcd-io/website/blob/main/content/en/docs/v3.6/tutorials/how-to-setup-cluster.md

Per https://github.com/etcd-io/website/issues/765, do the following:

- Add prerequisites (etcdctl and a currently running etcd cluster are required)
- Define all variables and flags used in the examples in a bulleted list
- Generally revise this article so it's a little easier to read through

When done, the document should have the following sub-sections (## H2 headings):

- Pre-requisites
+ Any subsections needed