Document high-availability setups

[zah]

Nimbus is compatible with the Blox staking SSV validator node which can be used to create highly-available redundant setups (where 1 out of 3 or 2 out 5 are allowed to fail, for example). In the future, it will also be compatible with the Charon middleware client from the Obol project. We need to document these configurations in a new "High-availability Setup" section in the Nimbus guide.

Besides fault-tolerance, these setups can bring all the advantages discussed in #3416.

[stefantalpalaru]

all the advantages

And the risk of getting slashed if there's a bug in your middleware, right?

[arnetheduck]

A related feature is the possibility to run several beacon nodes in a trusted mesh - this ensures that blocks and attestations get published via multiple touch-points in the mesh, and each node can independently be taken up and down - ie the idea is that we form private connections between these nodes and they each independently form public connections with the network.