Add a basic docc catalog

Sources/Queue/Documentation.docc/Queue.md

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+# ``Queue``
+
+<!--@START_MENU_TOKEN@-->Summary<!--@END_MENU_TOKEN@-->
+
+## Overview
+
+This package exposes a single type: ``AsyncQueue``.
+
+Conceptually, ``AsyncQueue`` is very similar to a ``DispatchQueue`` or ``OperationQueue``. However, unlike these an ``AsyncQueue`` can accept async blocks. This exists to more easily enforce ordering across unstructured tasks without requiring explicit dependencies between them.