Adds DocC catalog to host SwiftPM documentation (#8487)

Adds a `Documentation.docc` directory (a bare DocC catalog) as a stub
for migrating existing content in `Documentation/` from general markdown
into DocC, as well as providing a base for expanding that documentation.
Follow up PRs will bring over and convert content from the
`Documentation` directory more piecemeal

Author: heckj

Package.swift

@@ -429,6 +429,12 @@ let package = Package(
             ]
         ),
 
+        // MARK: Documentation
+
+        .target(
+            name: "PackageManagerDocs"
+        ),
+
         // MARK: Package Manager Functionality
 
         .target(
@@ -1043,6 +1049,8 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
         .package(url: "https://github.com/apple/swift-collections.git", "1.0.1" ..< "1.2.0"),
         .package(url: "https://github.com/apple/swift-certificates.git", "1.0.1" ..< "1.6.0"),
         .package(url: "https://github.com/swiftlang/swift-toolchain-sqlite.git", from: "1.0.0"),
+        // For use in previewing documentation
+        .package(url: "https://github.com/swiftlang/swift-docc-plugin", from: "1.1.0"),
     ]
 } else {
     package.dependencies += [

Sources/PackageManagerDocs/Documentation.docc/SwiftPackageManager.md

@@ -0,0 +1,14 @@
+# ``PackageManagerDocs``
+
+@Metadata {
+    @DisplayName("Swift Package Manager")
+}
+
+Organize, manage, and edit Swift packages.
+
+## Overview
+
+The Swift Package Manager is a tool for managing distribution of source code, aimed at making it easy to share your code and reuse others’ code. The tool directly addresses the challenges of compiling and linking Swift packages, managing dependencies, versioning, and supporting flexible distribution and collaboration models.
+
+## Topics
+