Documentation reorganisation

[Ian Clatworthy]

Following my RFC a few days ago, I've reorganised the material on our
main documentation overview page: http://bazaar-vcs.org/Documentation. I
believe this makes things much easier to find and will mesh nicely with
the migration work I'm doing moving core docs off the Wiki into the
development tree.

As mentioned earlier, the main gaps right now IMO remain:

* materials to make it easier for users to switch to Bazaar
* a few topics in the User Guide.

If anyone can help with these, please let me know.

How does the Documentation page now look to others? My goal is to get
the bundled doc updated to reflect the structure shown under Current
Documentation so please speak up if you have any feedback!

Ian C.