docs/pwg/: Add as a placeholder for future filling-in.

docs/pwg/: Add as a placeholder for future filling-in.

Original commit message from CVS:
2004-01-27  Ronald Bultje  <rbultje@ronald.bitfreak.net>

* docs/pwg/advanced_interfaces.xml:
* docs/pwg/pwg.xml:
Add as a placeholder for future filling-in.
* docs/pwg/basics_autoplugging.xml:
* docs/pwg/basics_buffers.xml:
* docs/pwg/basics_elements.xml:
* docs/pwg/basics_events.xml:
* docs/pwg/basics_plugins.xml:
* docs/pwg/basics_types.xml:
Remove, because unused (this is all in intro_basics.xml).
* docs/pwg/building_signals.xml:
Short intro to signals + reference to GObject docs - we really
shouldn't go into these sort of things to deply because we don't
use them that extensively anyway.
* docs/pwg/building_state.xml:
Explanation of states. Benjamin, please check.
* docs/pwg/building_testapp.xml:
Put everything in one page - putting only a few lines of content
per page doesn't really make sense.

Time to get into the advanced topics. ;).