docs: move GObject preface to its own file

And fix the indentation.

With a separate file, the content can more easily be reused, for example
to glue together chapters on GLib, GObject, GIO and GTK+ into a book.

https://bugzilla.gnome.org/show_bug.cgi?id=736914
docs: improve a little the GObject preface

&#8211; is a longer dash.

https://bugzilla.gnome.org/show_bug.cgi?id=736914
docs: improve the GObject introduction

Some of the added content comes from GGAD (for GtkObject).