Revise https://etcd.io/docs/v3.5/tutorials/how-to-watch-keys/

[celestehorgan]

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-watch-keys.md

Per #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

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

• Pre-requisites
• Watching keys