Add documentation for Multicast feature

[antoninbas]

trafficstars

Starting with Antrea v1.5, Antrea can support multicast traffic (within the cluster network, or between the Node network and the cluster network) under certain conditions. In order for users to understand how to use this feature, it needs to be documented.

[github-actions[bot]]

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[meibensteiner]

Is there some preliminary documentation on this? Especially interested in cluster network to node network communication.

[antoninbas]

Some documentation has been added: https://github.com/antrea-io/antrea/blob/main/docs/multicast-guide.md