docs: Add example config "template" for various levels of activity

A pre-defined config with various levels of activity already built in was much-requested on user calls.

MVP:
* Check in an `example_config.json` that has empty fields for node details (we can't pre-set this) and various different types of activity specified (high/low frequency, high/low amount etc).

We could extend this further to have different "profiles" like:
* `sink`: one node receives a lot of payments from the network
* `source`: one node sends a lot of payments to various destinations
* `popular`: one pair on nodes send/receive a high amount of traffic between themselves