openEO example showcasing MultiBackendJobManager feature

[Pratichhya]

Suggestion from @jdries:

for job manager notebook, perhaps we should consider the sample extraction use case. Maybe doing the best available pixel for sentinel-2, and then extracting samples for that over a full country or so.

We have a lot of code for that already, so would need to set something up to gather everything into a high-quality example.

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