Created the DocC documentation catalog in the library target.

Sources/DocCMiddleware/Catalogs/DocCMiddleware.docc/Library.md

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+# ``DocCMiddleware``
+
+<!--@START_MENU_TOKEN@-->Summary<!--@END_MENU_TOKEN@-->
+
+## Overview
+
+<!--@START_MENU_TOKEN@-->Text<!--@END_MENU_TOKEN@-->
+
+## Topics
+
+### <!--@START_MENU_TOKEN@-->Group<!--@END_MENU_TOKEN@-->
+
+- <!--@START_MENU_TOKEN@-->``Symbol``<!--@END_MENU_TOKEN@-->