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