Revised entire Design Center section.

This section has been heavily revised.

--Updated the index page. It serves as a useful starting point for people who want to know more about the Design Center and it provides a helpful roadmap.
--Created a "Deploy your first Policy" page for new Enterprise users who want to try a DC app tutorial.
--Created a new sub-topic called "Configure the Design Center App (Enterprise)" and added the following existing content. Note that all pages in this section have been edited.:
* Integrating the Mission Portal with Git   
* Controlling Access to the Design Center UI  
* Sketches Available in the Mission Portal           
* Sketch Flow in CFEngine Enterprise   

--Created a new page called "Configure Sketches on the Command Line (Community)" and moved instructions for installing/deploying sketches for Community users. This information was previously on the Index page. Also added "Advanced Walkthrough" as a sub-topic of this page.
--Created a new page called "Write a new Sketch." This is for Enterprise and Comm users.
--Added a new image to include on the index page. It accompanies a brief explanation of the Design Center workflow.