Port contents of Documentation/libSwiftPM.md to DocC (#8725)

Fixes #8594 

Port the contents of `Documentation/libSwiftPM.md` over to our DocC
catalog in
`Sources/PackageManagerDocs/Documentation.docc/SwiftPMAsALibrary.md`.

Author: bripeticca

Sources/PackageManagerDocs/Documentation.docc/Documentation.md

@@ -8,7 +8,7 @@ Organize, manage, and edit Swift packages.
 
 ## Overview
 
-The Swift Package Manager leets you share your code as a package, depend on and use other share packages, as well as build, test, document, and run your code.
+The Swift Package Manager lets you share your code as a package, depend on and use other share packages, as well as build, test, document, and run your code.
 
 ## Topics
 
@@ -23,6 +23,7 @@ The Swift Package Manager leets you share your code as a package, depend on and
 - <doc:AddingDependencies>
 - <doc:ResolvingPackageVersions>
 - <doc:CreatingCLanguageTargets>
+- <doc:SwiftPMAsALibrary>
 
 <!-- ### Command Plugins -->
 <!-- placeholder for content about swift package manager extensions - command plugins -->

Sources/PackageManagerDocs/Documentation.docc/SwiftPMAsALibrary.md

@@ -0,0 +1,15 @@
+# SwiftPM as a library
+
+Include Swift Package Manager as a dependency in your Swift package.
+
+## Overview
+
+> Warning: **The libSwiftPM API is _unstable_ and may change at any time.**
+
+SwiftPM has a library based architecture and the top-level library product is
+called `libSwiftPM`. Other packages can add SwiftPM as a package dependency and
+create powerful custom build tools on top of `libSwiftPM`.
+
+A subset of `libSwiftPM` that includes only the data model (without SwiftPM's
+build system) is available as `libSwiftPMDataModel`.  Any one client should
+depend on one or the other, but not both.