Add text on how SCTP restart works

[gloinul]

The solution should document how to perform SCTP Restart in a safe way. Based on what is written in Section 6.3 of RFC 4895 we believe a safe method for restarting is possible.

The restarting node need to have stored the SCTP-AUTH key that was in use prior to the restart. Then perform the INIT to initiate the restart per RFC 9260. Then use SCTP-AUTH with the old key in the Cookie-Echo message. That way proving to the peer that this is the same node. After that a new DTLS handshake can/must be done to re-authenticate the nodes on DTLS level, and then user messages can be sent using the new established DTLS connection and its derived SCTP-AUTH key.