Improve example with offline/online clients

[claustres]

Currently the example is only using frontends with automerge services. However, as stated in #10, this will not be practical for some use cases. It should be better to setup an example with:

• two servers, one to be sync with another one
• an "online" client directly connected to the adapter services of the sync server
• an "offline" client connected to automerge services of the synced server that can go offline

This will be representative of a use case where:

• most clients are connected to a central cloud server but some are connected to a locally deployed server to be sync with the cloud server
• some clients can go offline then sync back their changes when online again

[claustres]

If more practical we can also split the example into two examples:

• one for server-to-server sync
• one for client-to-server sync