Cool new easy-to-update Plugins Guide

[Ian Clatworthy]

I've spent some time over the last few days working on a shiny new
Plugins Guide. See http://doc.bazaar.canonical.com/plugins/en/.

Major improvements are:

* New plugins added including builder and externals. A new category
  called "Metaprojects" was added for these.

* The "Patch management" category was renamed to "Change ordering" to
  reflect the broader problem domain. The "undelete" plugin was added
  here to accompany loom, pipeline and rebase.

* New topics on:
  - Installing a plugin
  - Developing a plugin (a nice new tutorial)

* Content refreshed and a shiny new header for existing plugins,
  including links to the branch, home page and owning user/team.
  Platform support information is now included here too.

Most important of all, this version in now part of bzr-alldocs meaning
that it gets rebuilt nightly with any changes committed to that project.
As a result, getting new plugins added or existing content refreshed is
now much easier. Hooray!

There's still more to do here, e.g. adding an introduction section for
each category to help users decide which plugin is best when, adding
many more plugins, polishing the development tutorial and generating the
"registry" in the wiki from the better one now inside alldocs. Feel free
to submit merge proposals for any of these and other changes you'd like
to see. (I won't be working on these in the near future fwiw.)

Enjoy,
Ian C.

PS: Poolie, please tweak the symlink on escudero to point to the
generated content (under alldocs/_build/plugins/en) instead of my local
copy.