Add Hierarchical grid both root and row-island lv paginator template code snippet setup in documentation

[IvayloG]

1. Show how to set paginator on root grid lv
   <igc-paginator per-page="${5}"></igc-paginator>

2. Show how to set paginator on row-island level

public rowIslandPaginatorTemplate = () => {
  return html`<igc-paginator per-page="${7}"></igc-paginator>`;
}

<igc-row-island .paginatorTemplate="${this.rowIslandPaginatorTemplate}">