Revamp the Snapshots section

[smcvb]

The current Event Snapshots page contains some inconsistencies in the explanation.
For one, it assumes the AggregateSnapshot to be the alternative of custom snapshot events, while that's no longer the case.
Added, the code sample shows Axon-2 code that's no longer present in more recent versions of the framework.

Especially the custom snapshot events description would benefit from a more thorough sample to guide users.
In the wake of JDK17 and changes in serialization, it's likely becoming more reasonable for end-users to utilize this capability.
As such it would be good to take a careful look at the entire section and make it more up-to-date.