docs/manual/: Add scale factor for pdf output.

docs/manual/: Add scale factor for pdf output.

Original commit message from CVS:
patch by: Luc Pionchon  <luc.pionchon@nokia.com>
* docs/manual/advanced-autoplugging.xml:
* docs/manual/advanced-threads.xml:
* docs/manual/basics-bins.xml:
* docs/manual/basics-elements.xml:
* docs/manual/basics-helloworld.xml:
* docs/manual/basics-pads.xml:
Add scale factor for pdf output.
* docs/manual/intro-basics.xml:
Switched sections "pads" and "bins" and added a pipeline diagram.
* docs/manual/intro-gstreamer.xml:
Added more info on gstreamer.
* docs/manual/intro-motivation.xml:
Commented out the whole section "current problem", which sounds
historical and somehow osolete; it could be turned in a positive
way and reused to improve the design principles.
* docs/manual/intro-preface.xml:
- Update URLs to library.gnome.org.
- Do not mention GTK+ in preliminary reading (irrelevant).
- Mention Plugin Writer's Manual and further reading only in the
previous section.
- Added a list of most relevant GObject/glib topics.
* docs/manual/Makefile.am:
* docs/manual/bin-element-ghost.fig:
* docs/manual/bin-element-ghost.png:
* docs/manual/bin-element-noghost.fig:
* docs/manual/bin-element-noghost.png:
* docs/manual/bin-element.fig:
* docs/manual/bin-element.png:
* docs/manual/filter-element-multi.fig:
* docs/manual/filter-element-multi.png:
* docs/manual/filter-element.fig:
* docs/manual/filter-element.png:
* docs/manual/gstreamer-overview.png:
* docs/manual/hello-world.fig:
* docs/manual/hello-world.png:
* docs/manual/linked-elements.fig:
* docs/manual/linked-elements.png:
* docs/manual/mime-world.fig:
* docs/manual/mime-world.png:
* docs/manual/queue.fig:
* docs/manual/queue.png:
* docs/manual/simple-player.png:
* docs/manual/sink-element.fig:
* docs/manual/sink-element.png:
* docs/manual/src-element.fig:
* docs/manual/src-element.png:
* docs/manual/diagrams-general.svg:
* docs/manual/diagrams-pipelines.svg:
Removed .fig, added .png counterpart.
Fixes: #539137