Add a section about the new Location management tool

[giohappy]

A new section must be created inside the User manual to explain how the location tool works (see https://github.com/GeoNode/geonode-mapstore-client/issues/1539):

• The tool is available as a new "Location" tab inside the preview and information panels
• The preview panel shows a read-only version of the tool, showing the extent and location of the resource
• The info panel inside the detail page of a resource shows the editable tool:
  • Shift+click and drag to draw a new extent
  • Click the marker and drag to change its location
• Extent and location cannot be edited for datasets and maps, since the underlying spatial data implicitly define them