docs/plugins/: Add twolame and celt plugins to the docs.

docs/plugins/: Add twolame and celt plugins to the docs.

Original commit message from CVS:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-bad-plugins-docs.sgml:
* docs/plugins/gst-plugins-bad-plugins-sections.txt:
* docs/plugins/gst-plugins-bad-plugins.args:
* docs/plugins/gst-plugins-bad-plugins.hierarchy:
* docs/plugins/gst-plugins-bad-plugins.interfaces:
* docs/plugins/gst-plugins-bad-plugins.prerequisites:
* docs/plugins/inspect/plugin-celt.xml:
* docs/plugins/inspect/plugin-twolame.xml:
Add twolame and celt plugins to the docs.