Invoke soong_docs from the bootstrap Ninja file.

This makes soong_ui the only place where soong_build is invoked, thus
greatly simplifying the conceptual model of the build.

It comes with the slight limitation that now soong_docs (and queryview
and the JSON module graph) are not Make targets anymore, but I suppose
that's an acceptable loss.

The only place where someone depended on soong_docs from a Makefile is
removed in a separate change.

Test: Presubmits.
Change-Id: I3f9ac327725c15d84de725d05e3cdde1da3dcbe2

android/Android.bp

@@ -80,7 +80,6 @@ bootstrap_go_package {
         "util.go",
         "variable.go",
         "visibility.go",
-        "writedocs.go",
     ],
     testSrcs: [
         "android_test.go",