Grid performance demo with big data is missing

[limonte]

One of important vaadin-grid's features is performance. We need to demonstrate it.

The demo example of big data with maximum features enabled - filtering, sorting, column reordering, etc. (here's ag-grid's demo: https://www.ag-grid.com/example.php#/performance/1)

[web-padawan]

Moved the issue to the docs repo as the examples are now located here.