documentation: organize top-level page better

[jack-oquin]

In my opinion, the top-level catkin documentation page has too much verbiage before new users get to see how the information is organized.

Users coming to catkin for the first time generally want to quickly find out how to do some specific task.

• Since they are probably forced to use catkin, they don't need to know its key benefits right away.
• For most, catkin is already installed with the rest of ROS, so only a few need that information, though it should be readily accessible early the index.

Please assign this to me, if possible. I'll write a draft to submit for review.

[jack-oquin]

The second-level sections needed some reorgainzation, too:

• Moved from "User Guide" to "Advanced Users Guide":
  • "How to build the docs"
• Moved from "Index" to "User Guide":
• "Installation"
• Moved from "Advanced Users Guide" to "User Guide":
• "Variables"
• "Glossary"