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

[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-access-etcd.md

Per #765, do the following:

1. Add prerequisites (etcdctl is required)
2. Define all variables used in the examples in a bulleted list (in this case, $ENDPOINTS, foo and `"Hello world"

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

• Pre-requisites
• Write to an etcd cluster
• Read from an etcd cluster