Update PivotTable examples

[lovtsovaik]

Environment

• Documentation version: 7.2
• Part of the documentation: Charts
• Existing page: https://doc.cuba-platform.com/charts-7.2/charts.html#pivotTable

Description of the bug or enhancement

§4.1.3 Editable PivotTable
The error in code:

    <chart:aggregations default="COUNT">
        <chart:aggregation caption="Count"/>
        <chart:aggregation mode="SUM_OVER_SUM"/>
    </chart:aggregations>

Should be replaced with

    <chart:aggregations default="COUNT">
        <chart:aggregation mode="COUNT"/>
        <chart:aggregation mode="SUM_OVER_SUM"/>
    </chart:aggregations>

because the mode attribute contains possibly options, and the default attribute should be one of them. Otherwise, the opened table is empty and the default parameter is ignored. This can also be added to the documentation text.

§4.3 Using ShowPivotAction
There is not enough information to understand how to use the action.

It needs to clarify that ShowPivotAction should be imported as
com.haulmont.charts.gui.components.action.list.ShowPivotAction, not as com.haulmont.charts.gui.components.action.ShowPivotAction (old implementation)

Also update and expand the example

ShowPivotAction showPivotAction = new ShowPivotAction(tipsGroupTable);
exportButton.setAction(showPivotAction);

to

@Subscribe
public void onInit(InitEvent event) {
    ShowPivotAction showPivotAction = actions.create(ShowPivotAction.class);
    showPivotAction.setTarget(tipsTable);
    defaultShowPivotBtn.setAction(showPivotAction);
}

It would be helpful to clarify how to get the components that extend ListComponent.

@Inject
private GroupTable<TipInfo> tipsTable;

Add the good example link https://demo10.cuba-platform.com/sampler/#main/3/sample?id=pivot-table-show-pivot-action