[GTK] Streamline wk1 documentation build

[GTK] Streamline wk1 documentation build
https://bugs.webkit.org/show_bug.cgi?id=70447

Reviewed by Martin Robinson.

.:

* GNUmakefile.am: include the documentation's GNUmakefile.am;
* autogen.sh: we no longer call gtkdocize, since we now ship
  a modified gtk-doc.make;
* configure.ac: no longer generate a separate GNUmakefile for
  the documentation build.

Source/WebKit/gtk:

* GNUmakefile.am: move files pertaining to the docs build to the
  docs GNUmakefile.am;
* GNUmakefile.gtk-doc.make: incorporate the gtk-doc makefile, and customize it
  to our non-recursive build;
* docs/GNUmakefile.am: simplification and adaption to work with the
  rest of the build system.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@97901 268f45cc-cd09-0410-ab3c-d52691b4dbfc