[data grid] Remove onCellFocusOut and add documentation for alternative recommended approach.

[joserodolfofreitas]

Summary 💡

Initially, we considered adding onCellFocusIn, to complement the use case of onCellFocusOut. The internal event cellFocusIn is already in place, so that'd mean just adding a callback API.

It turns out users can't rely on these events to properly track focus on the Data Grid cells.
The recommended approach is through componentProps, like in this codesandbox.

Motivation 🔦

Enables new extensions on the data grid behavior.
It could help on issues like:

• [data grid] Is there capability to have Formula Row, which presents value/text of the selected cell for ease of Row, like Excel/Google Sheet? #6267