Revise https://etcd.io/docs/v3.5/tutorials/how-to-deal-with-membership/

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-deal-with-membership.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 (in this case, `--endpoints, `member remove` and `member add`)
- Link to https://etcd.io/docs/v3.5/learning/design-learner/ so that readers can learn a bit more about members in etcd if needed

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

- Pre-requisites
- Add members
- Remove members