Revise server port docs

[adamnovak]

This should fix #476 by changing the docs to point at the port we actually run the server from npm run serve on, and documenting how to move it on the command line and why you might need to.

I wanted to move the npm run serve user-facing port you actually need to browse to to be 3001, the same as the npm run start one, but I couldn't work out how to do that while respecting config.json's serverPort and not needing new config for the demo deployment.