Add documentation on egress redundancy

[zonque]

No description provided.

[zonque]

I do have a similar approach for ingress redundancy, but I didn't get around to to test it yet. If anyone is willing to try, I'm happy to share the notes I jotted down.

[bondagit]

I have tested it and it works.
The required changes to the driver are now merged into the driver version v1.8
To use the egress net filter configuration a kernel version >= 5.16.x is required and config option CONFIG_NETFILTER_EGRESS should be enabled.
To load the configuration you provided I used:
sudo nft -f <egress.conf>
Active net filter rules can be dumped with:
nft list ruleset
In order for the daemon to run on the ravenna-dummy interface an IP address should be assigned to it.
Of course the PTP master traffic is still expected to be received via the above interface.
Anyway, since at the moment it's not possible to create a source with a custom SDP I am not going to merge this into the daemon main.