coroutines-guide.md

The main coroutines guide has moved to the [docs folder](docs/topics/coroutines-guide.md) and split up into smaller documents.

## Table of contents

<!--- TOC_REF docs/topics/basics.md -->
<!--- TOC_REF docs/topics/cancellation-and-timeouts.md -->
<!--- TOC_REF docs/topics/composing-suspending-functions.md -->
<!--- TOC_REF docs/topics/coroutine-context-and-dispatchers.md -->
<!--- TOC_REF docs/topics/flow.md -->
<!--- TOC_REF docs/topics/channels.md -->
<!--- TOC_REF docs/topics/exception-handling.md -->
<!--- TOC_REF docs/topics/shared-mutable-state-and-concurrency.md -->
<!--- TOC_REF docs/topics/select-expression.md -->
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