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

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-conduct-elections.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`, `elect`, and `p1`/`p2`)

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

- Pre-requisites
- Conduct leader election