Migrate PackageDescription.md reference (#8675)

The content for `Documentation/PackageDescription.md` is currently
hosted on https://docs.swift.org/package-manager/ and is considered
legacy content to be removed. The newer version of this documentation is
hosted at https://developer.apple.com/documentation/packagedescription
for which the content is found under
`Sources/PackageDescription/PackageDescription.docc/PackageDescription.md`.

We'd like to direct users to this developer documentation from our DocC
catalog, but since DocC does not currently support external linking to
other DocC catalogs we can simply create an article in our catalog that
provides a redirect link to above link.

Fixes #8590

Author: bripeticca

Sources/PackageManagerDocs/Documentation.docc/Documentation.md

@@ -27,8 +27,8 @@ The Swift Package Manager leets you share your code as a package, depend on and
 <!-- - <doc:swift-docc-plugin> -->
 <!-- - <doc:swift-container-plugin> -->
 
-<!-- ### Reference -->
-<!-- link to PackageDescription API reference docc -->
+ ### Reference
+- <doc:PackageDescription> <!-- redirect page to PackageDescription API reference docc -->
 <!-- link to Command API reference docc - the DocC Plugin API (PackagePlugin) -->
 
 <!-- reference content for the CLI commands `swift ...` -->

Sources/PackageManagerDocs/Documentation.docc/PackageDescription.md

@@ -0,0 +1,5 @@
+# Package Description
+
+Create reusable code, organize it in a lightweight way, and share it across your projects and with other developers.
+
+For more information about the `PackageDescription` framework, please visit the [API documentation](https://developer.apple.com/documentation/packagedescription).