[HxGrid] [doc] Progress indicator demo vs. scrolling

[hakenr]

We think the scenario where the progress indicator should be shown (i.e., a long-running operation > HxGrid.ProgressIndicatorDelay (default is 300ms)) and the header is scrolled away from the viewport is quite rare, so we decided to sacrifice it for now (the previous version was suffering from the same issue).

We should definitely fine-tune the demo, probably with sticky-top to avoid this issue.

Originally posted by @rqx110 and @hakenr in #842 (comment)