Add example for Percentile calculation in Example Queries page

[simranquirky]

• Percentile P95 P99

  SELECT histogram(_timestamp) as x_axis_1,  
      approx_percentile_cont(duration, 0.95) as percentile_95, 
      approx_percentile_cont(duration, 0.99) as percentile_99 
  FROM default 
      where service_name='$service' 
      GROUP BY x_axis_1 ORDER BY x_axis_1

add sample use case for this in example queries page