Update training module for forestry to use native point cloud tools

[saberraz]

The current documentation points to use of LasTools for viewing and generating DEM for forestry data:
https://docs.qgis.org/3.34/en/docs/training_manual/forestry/basic_lidar.html#lesson-dem-from-lidar-data

Now that we have native tools to view and process point clouds, this chapter can point to those tools instead of using a third party tool.