Add documentation for running grid getSelectedRows() api

[masykur8d]

As written in the current jutspy documentation, selected rows value are collected using grid.on('rowSelected', row_selected4). If we enable checkboxSelection in the column header and check it, rowSelected event will fire as many as the number of rows. I think this is fine if we only have small number of rows as shown in the documentation example. In my case, I have many grids and more than 3000 rows for each grid. When I select the header check box, it will take some time to finish and will block the entire process of application as I mentioned here. In addition, the page is prematurely reloaded automatically without warning at some point.

Rather than using grid.on('rowSelected', row_selected4), I think it is better to use grid api getSelectedRows().
Example of this api looks like below:

first, we run the api by using run_javascript method that can be found here. For me, I bind this to button before event.

async def selected_rows(self, msg):
        await jp.run_task(msg.page.run_javascript(f"""cached_grid_def['{self.grid.grid_id}'].api.getSelectedRows()""", request_id='grid_1_selection')) 

Second, we collect the result of the getSelectedRows() api by using:

async def resultReadyGridSelection(self, msg):
        if msg.request_id == 'grid_1_selection':
            self.grid.selectedRow = msg.result
        await asyncio.sleep(1)
wp.on('result_ready', resultReadyGridSelection)

This way, the rows selected value are collected only once thus not blocking the process.