Write a separate section in documentation for proxy queries and examples for custom ones

[Kreyu]

Proxy queries are the hearts of tables, since they work like an adapter between data source and a data table. Currently, those classes are explained in the extensibility section, which is kinda hidden.

It should be moved to a separate section with examples of custom proxy queries integrating with external APIs, like this one: https://jsonplaceholder.typicode.com/guide/