KV sharding and loadbalancing are not covered in docs

A user was asking about how requests are loadbalanced and what sharding is used. I'm flabbergasted to find that we don't include that in any of our contemporary documentation. It looks to be in the 3.x docs and I know it's in many old presentations, but not at all in the docs. I didn't have anything to point the user to.

I feel like this should be described conceptually.

I did find https://developer.couchbase.com/documentation/server/current/concepts/distributed-data-management.html with some more searching, but it's in "Client Topology Awareness" and while accurate, gives details without describing the concept.

A further search turned up this document from the 3.x days: http://www.couchbase.com/binaries/content/assets/website/docs/whitepapers/couchbase-server-under-the-hood.pdf