Write a Datastore Overview doc

[jgeewax]

We don't have a summary doc on rtd.org... It'd be really nice ot have an overview for Datastore.

[dhermes]

@lukesneeringer This still doesn't exist

[lukesneeringer]

I know, but I am going to be doing a larger rehash of docs with @omaray, so do not need an issue to track Datastore specifically.

[dhermes]

👍