Documentation improvements - feedback

[Valentin Aitken <va...@cloudsoftcorp.com>]

As a new user, this week I had to use the documentation to setup 
brooklyn and get into it.
It was a little hard for me to navigate through it and I think you also 
agree that there is stuff that could be improved, so it could be easier 
to find the information needed and increase the general usability of the 
website.

I made two pull requests which I'd love if you review and get feedback:

 1. implemented hierarchical menu for the guide for better navigation
    through the documentation -
    https://github.com/apache/incubator-brooklyn/pull/519
 2. include the header menu in the catalog, remove obsolete repetitions
    and simplify procedure for adding a new entity -
    https://github.com/apache/incubator-brooklyn/pull/524

Please review them and write your feedback in the pull request or by email.

Kind Regards,
Valentin Aitken!