Add example on how to bootstrap a cluster via v3 discovery

Add an example into the doc https://etcd.io/docs/v3.6/dev-internal/discovery_protocol/ to help users understand the process

Start by reading the v3 discovery protocol documentation at https://etcd.io/docs/v3.6/dev-internal/discovery_protocol/ and identify where an example best fits. Add an example that explains how to bootstrap a cluster via v3 discovery, then verify that the documented process is understandable and consistent with the surrounding page.

Written by the indexing model from the issue text.