Add WCS default bands to WCS user guide

[eefaye]

Imported WCS layers currently do not show band names, instead defaulting to Band1, Band2 etc. This seems to be a QGIS problem:

band names are embedded in the geotiff metadata but QGIS doesn't seem to read them.

This is particularly unhelpful to GIS users who have no way of knowing which bands are which.
The default bands and order for each service are defined in the back-end config files on digitalearthafrica/config.

It would be helpful if the WCS Guide on the Docs website could have a table detailing the band values for each data service, for users to refer to.

[eefaye]

This may possibly be obsolete with the latest WCS update which removes the default band parameter and assumes all bands if not specified in the query.