docs: add Grid Data Binding page

[vursen]

Related to vaadin/flow-components#1886

[sissbruecker]

We do we add another page that explains data provider / data view concepts when there is already one in the Flow docs with the same content? Why not improve the existing one?

[vursen]

It's a bit hard to discover right now IMO. The new page doesn't aim to explain everything but to give a brief introduction specific to Grid and refer to the Flow documentation for more in-depth information.

I agree that at least "Custom Data Providers" would be better placed in the Flow documentation. And I could probably remove the setItems(fetchCallback, countCallback) example to keep things simpler.

[vursen]

The intention was also to give complete, copy-pasteable Grid-specific examples that work right away.

[vursen]

Another goal was to make both Grid and Tree Grid have a Data Binding page so that their documentation structure is more consistent.

[sissbruecker]

OK, makes sense. Then I'd suggest to keep both the setItems examples and move the custom data provider example to the more in-depth Flow reference.

[vursen]

I moved some of the content to the Flow reference, though I still think that a dedicated Grid data provider example is worth having on the Grid's Data Binding page at least for now. The Flow's Data Binding article is more focused on Spring usage and its examples aren't directly copy-pastable.