Set a custom display name for the plugin's documentation (#16)

With Swift 5.7, Swift-DocC has support for setting a custom display name
in documentation. We can use it here to render the top-level page of
the plugin's documentation as "Swift-DocC Plugin" instead of "SwiftDocCPlugin".

Author: ethan-kusters

Sources/SwiftDocCPluginDocumentation/SwiftDocCPlugin.docc/SwiftDocCPlugin.md

@@ -2,6 +2,10 @@
 
 Produce Swift-DocC documentation for Swift Package libraries and executables.
 
+@Metadata {
+    @DisplayName("Swift-DocC Plugin")
+}
+
 ## Overview
 
 The Swift-DocC plugin is a Swift Package Manager command plugin that supports building