Add editable data grid section to docs (#499)

Description of changes

Adds a new section to the data grid docs explaining the current state of the world for editable data grids and provides a link to the editable data grid sample extension.

Author: hawkticehurst

src/data-grid/README.md

@@ -302,3 +302,11 @@ basicGrid.columnDefinitions = [
   { columnDataKey: "ColumnKey4", title: "Custom Title" },
 ];
 ```
+
+## Editable Data Grid
+
+Interactive/editable data grids are a highly requested feature in the Webview UI Toolkit.
+
+By default, the toolkit and in particular [FAST](https://www.fast.design/) (the underlying framework that the toolkit uses) **do not** offer any first-party APIs for enabling interactivity or editability in data grids. Additionally, since FAST owns the underlying data grid implementation, first-party support for interactivity will eventually need to come from them.
+
+With that said, workaround solutions are possible and a reference implementation for making the `vscode-data-grid` component editable is [available as a sample extension](https://github.com/microsoft/vscode-webview-ui-toolkit-samples/tree/main/default/editable-data-grid).