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/Android.bp b/Android.bp
index 643e8df..ebf1038 100644
--- a/Android.bp
+++ b/Android.bp
@@ -61,6 +61,7 @@
         "android/testing.go",
         "android/util.go",
         "android/variable.go",
+        "android/writedocs.go",
 
         // Lock down environment access last
         "android/env.go",