Explain how to upgrade NGF with zero downtime

[pleshakov]

Explain any necessary steps, configuration options or notes for the cluster operator to be aware to perform upgrades of NGF with zero downtime, which means clients

WebSockets and long lived connection is a special case -- https://github.com/nginxinc/nginx-gateway-fabric/blob/cd8ba03cb5d0c763133fe8b04127be1e4b460826/tests/zero-downtime-upgrades/zero-downtime-upgrades.md#non-goals

Acceptance criteria:

• Add documentation in appropriate places
• Provide a note on WebSockets and long lived connections.

Blocked by #1155 #1143