Use module symbol title, add DisplayName and abstract to landing page (swiftlang#48)

Switch the SwiftAndroid landing page from a plain-text heading to the
module symbol reference so its abstract and content attach to the
module page. Add a DisplayName of "Swift for Android" and a concise
abstract.

Author: heckj

Sources/SwiftAndroid/SwiftAndroid.docc/SwiftAndroid.md

@@ -1,6 +1,10 @@
-# Swift on Android
+# ``SwiftAndroid``
 
-Use Swift to build native libraries and applications for Android
+Build native libraries and apps for Android with Swift, from toolchain setup to integrating with Android tooling.
+
+@Metadata {
+    @DisplayName("Swift for Android")
+}
 
 ## Topics