Create a notebook to demonstrate accessing the expanded extents data

[BexDunn]

we can make a demonstration notebook for the expanded extents data and how to handle it

[robbibt]

I reckon this could even just be a small extra bit at the end of the existing Landsat and Sentinel-2 ARD notebooks - my understanding is that the data should be generally usable in the same way as before, but there might be some extra caveats and important things to keep in mind (have been planning to ask @uchchwhash to organise a presentation on the Expanded Extents project outcomes so we can all understand those caveats/limitations/changes better).