SDK3 - Advanced Working with Data section: Async, Concurrent, Batching

Advanced Working with data section for SDK3

 - see Sub-tasks for content.

(Or refer to [info architecture|https://docs.google.com/document/d/1f0N8ZxUmE7BlnVanRubUe8hR-epCr62hAwaPeKMknD4/edit#heading=h.f2iacrpl339b)

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Update – Async & Batching pages updated:
Will both appear in a single page, covering choices of asynchronous and reactive APIs:
https://github.com/couchbase/docs-sdk-scala/blob/release/1.0alpha/modules/howtos/pages/concurrent-async-apis.adoc

Purpose of the doc is to illustrate using the various choices (blocking/reactive/async/bulk ops), with emphasis on the best choice idiomatic to the platform/language. No central template, as each SDK will be different.

GitHub repos have the templates to be filled (check the source, the comments won't be rendered in the links):
https://github.com/couchbase/docs-sdk-c/blob/release/3.0/modules/howtos/pages/concurrent-async-apis.adoc
https://github.com/couchbase/docs-sdk-dotnet/blob/release/3.0alpha/modules/howtos/pages/concurrent-async-apis.adoc
https://github.com/couchbase/docs-sdk-go/blob/release/2.0alpha/modules/howtos/pages/concurrent-async-apis.adoc
https://github.com/couchbase/docs-sdk-java/blob/release/3.0alpha/modules/howtos/pages/concurrent-async-apis.adoc
https://github.com/couchbase/docs-sdk-nodejs/blob/release/3.0/modules/howtos/pages/concurrent-async-apis.adoc
https://github.com/couchbase/docs-sdk-php/blob/release/3.0/modules/howtos/pages/concurrent-async-apis.adoc
https://github.com/couchbase/docs-sdk-python/blob/release/3.0/modules/howtos/pages/concurrent-async-apis.adoc
https://github.com/couchbase/docs-sdk-scala/blob/release/1.0alpha/modules/howtos/pages/concurrent-async-apis.adoc