A User Guide page on creating a Cube from scratch

[trexfeathers]

📚 Documentation

Creating a Cube from scratch is valuable in many contexts, but many of our users are unaware that this is even an option (believing instead that they can only be created via loading). We should advertise that this is possible, and how to do it.

[pp-mo]

Reference : In the peloton meeting this morning it occurred to me that this issue was worth another mention, in the context of the #4447 : for example. how can you create an abstract target grid for regridding to