Document best practice around Sentinel-2 `contiguity` bands and resampling

[robbibt]

Sentinel-2's contiguity band is processed at 10 m, but in theory covers other lower resolution bands too. This means that when loading the band at 20 m resolution, care must be taken to use the correct resampling method to get sensible results.

For example, since a 20 m pixel can contain a mix of contiguous and non-contiguous 10 m pixels, it should always be loaded using "minimum" resampling (so that if one 10 m pixel is non-contiguous, the entire 20 m pixel is).

We should document this in the Sentinel-2 notebook so users can make an informed choice around resampling contiguity data.