[Fleet] Document how routing rules can be used

Integrations will be able to define routing rules about how data from other integrations or data streams should be routed to their own data stream. For example, the nginx package may define a routing rule for the logs-kubernetes.container_logs data stream to route logs to logs-nginx.access whenever container.image.name == "nginx". Similarly, when the kubernetes package is installed and the nginx was also previously installed, we'll need to ensure the logs-kubernetes.router-{version} ingest pipeline includes a reroute processor for each routing rule defined on the nginx integration.

To support this, we added a [concept of routing rules to the package spec](https://github.com/elastic/package-spec/issues/514) and we will add [support for them in Fleet](https://github.com/elastic/kibana/issues/155910).