Add page about testers API

[edgarmueller]

No description provided.

[sdirix]

Add a new doc page for testers. Explain the tester architecture of JSON Forms. Show the Raw API and which utilities are provided by JSON Forms, especially rankWith, schemaMatches, uiTypeIs, optionIs, formatIs, and, or.

Do this with many code examples (don't need to be demoed).

Explain for an example schema and ui schema (you can orientate yourself on the Rating example) how the custom renderer could be registered only for the specific property, for all numbers or based on a ui schema indicator.