[2.0.x] Add documentation for devs

Work has started

Also add:

We are converted data types from the more specific to the less specific. For example:

• An URL data source can be used in a string prop because every URL is also a string. However, a string data source can't be used in an URL prop because not every string is an URL
• A string data source can be used in a slot because every string is also a renderable. However, a free renderable source (like ViewField) can't be used in an string prop because not every renderable is a string.