Allow Soong to generate its own build documentation

Add a rule in soong that re-executes soong_build in order to
generate build documentation.  This allows Soong to customize
the documentation.

Bug: 70516282
Test: m soong_docs
Change-Id: If143cfacd6ac20274cd7bb8d8fab0c07025a5553
diff --git a/cmd/soong_build/Android.bp b/cmd/soong_build/Android.bp
index d9daafc..2536a53 100644
--- a/cmd/soong_build/Android.bp
+++ b/cmd/soong_build/Android.bp
@@ -23,6 +23,7 @@
     ],
     srcs: [
         "main.go",
+        "writedocs.go",
     ],
     primaryBuilder: true,
 }