doc: revise and augment user guide

[meling]

Make progression of different examples ranging from

1. Simple gRPC example
2. Load balancing accessing two distinct services
3. Distributed storage with simple majority quorums (both single-writer and multi-writer)
4. Distributed storage with byzantine quorums
5. Advanced protocol: Paxos with majority quorum??
6. Advanced quorum functions with correctables (can we use EPaxos as an example, or could we do something else?)
7. Advanced quorum functions with per_node_arg and custom_return_type

(consider splitting this issue into a separate issue for each example.)

[meling]

The examples above should also describe how to organize your repos, and how to use the vendor folder. Moreover, we should also describe an advanced example where you want to extend Gorums as a library like was done for the Raft implementation (@s111).

[meling]

The load balancing example can benefit from this blog post. Perhaps we could specify a bachelor thesis for implementing the various LB alternatives described in the blog post.