Remove redundant PackageDescription docs page (#9237)

Since we now pull in the API reference for `PackageDescription` (see
https://docs.swift.org/swiftpm/documentation/packagemanagerdocs/packagedescription/),
it's redundant to keep the extra page we had in SwiftPM that links to
the
[developer.apple.com](https://developer.apple.com/documentation/packagedescription)
docs.

Author: bripeticca

Sources/PackageManagerDocs/Documentation.docc/Documentation.md

@@ -43,10 +43,6 @@ The Swift Package Manager lets you share your code as a package, depend on and u
 <!-- - <doc:swift-docc-plugin> -->
 <!-- - <doc:swift-container-plugin> -->
 
- ### 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 ...` -->
 ### Swift Commands