Document how to create a custom Modal

[maxime-rainville]

By default, all DataObject using the AnyField, will use the AnyModal to render the form.

If a DataObject had more complex needs, they have the option to use some alternative Modal.

• One example of this is a scenario where you need to select a file and render the form schema inside a InsertMediaModal.
• Another scenario could be where a user needs to interact with some third party system and provide custom functionality in the modal. Say maybe, you need to collect some Google Map data and add some extra meta data on top of it.

The facility for achiving this is already built into the system. But the usage, is not documented.

Acceptance criteria

• The doc provides clear guidance for constructing custom AnyField Form modals along with some concrete examples.

[maxime-rainville]

Arguably, this is an advanced feature that is not needed in the official release.