Cosmos: Create a compelling sample

[divega]

As we make progress with our provider, we want to build a sample that does a good job of showcasing the benefits of Cosmos DB's SQL API at the same time that it takes advantage of the EF Core programing model.

One idea that came up was to find a good existing Cosmos DB sample and adapt it to use our provider.

[divega]

For now the closest thing we have to a sample is a demo at https://github.com/divega/EFDemos/tree/master/cosmos-db.

[AndriySvyryd]

Tips:

• Partition key should be part of the PK

[JeremyLikness]

@ajcvickers @AndriySvyryd is this something we can pull in an existing demo like https://github.com/jeremylikness/planetarydocs for, or just reference to it? Or do we want something custom "ground up" for docs? The planetary demo shows fromsql, multiple containers, primitives, implicit ownership, etc.

[ajcvickers]

@JeremyLikness I think an existing demo is fine, but maybe we should consider moving a copy into our samples so that we can reference code from it in the normal way.