Update binary format to mention server_definitions method

[mDuo13]

The binary format docs mention the definitions file, but now you can query the definitions from the server using the server_definitions method, which returns the same format as the JSON file in the xrpl.js repo that the docs currently link to.

This is a bit easier especially if you're updating a client library to use features and fields that are in development, since you can query a dev server for its working version of the definitions file. Since authors of client libraries are the main target audience for the binary format documentation, we should definitely update the binary format page to mention the server_definitions method.