Give clearer examples of loading PP files

Cube_helper was designed to help with loading netCDF from CMIP and so it assumes that each file has an 'nc' suffix. To load PP files something like:

test2 = ch.load([file], 'pp')

is required. This is documented at https://cube-helper.readthedocs.io/en/v2.2.3/Cube%20Helper%20Module.html#cube_helper.load but isn't clear in the Quickstart or Tutorial. A quick example should be added to these two sections of the documentation.

Start by reading the existing cube_helper.load documentation and then compare the Quickstart and Tutorial sections. Add a concise example showing how to load a PP file with the required format argument in both sections. Done means readers can find and understand the PP-loading example in each location.

Written by the indexing model from the issue text.