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

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-migrate.md

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

- Add prerequisites (etcdctl and a currently running etcd2 cluster are required)
- Define all variables and flags used in the examples in a bulleted list (in this case, `--endpoints, `set`, `--output`, `get`, `--data-dir`, `--wal-dir`)

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

- Pre-requisites
- Migrate a cluster