Gedit docs shouldn't mention extra plugins

[Mario Blättermann <mario blaettermann gmail com>]

Hi,

while translating the Gedit documentation I've seen that a lot of
plugins are part of the main manual which are _not_ part of the gedit
main package. This forces translators first to make sure that they have
all the GUIs of the appropriate plugins translated (which are in general
not part of the GNOME core release set).

Mallard has the ability to mix docs from different sources. Means, we
should actually move the plugin docs to the plugin packages
(gedit-plugins and perhaps some other from third party vendors. This is
not my first attempt to do so. Removing the plugin topics from the base
manual would it make easier to maintain the docs anyway, because we
don't have to mention things such as "If the xxx package is not
installed, you will be unable to use this feature".

Of course, it's too less time to get it into GNOME 3.4, but we should
keep an eye on 3.6.

Best Regards,
Mario