Document the Neo4j Harness library for testing unmanaged extensions

This is the follow up to the PR that introduced the neo4j-harness component. That component is meant to be a designed-for-public-consumption replacement of ServerBuilder, and this PR contains the public documentation for it.
See here for how to get the actual version included instead: https://github.com/neo4j/neo4j/blame/master/community/embedded-examples/src/docs/dev/setup.asciidoc#L63