Getting Started Guide revision from Margaret

I made a lot of changes to the Getting Started Guide (getting_started.rst) which are described in the comments from my last commit.  Let me know what you think.

Also, I see that you added the navigation. Looks good.  I have a lot of suggestions for improving the architecture and the content.  This repository doesn't seem to have the latest content. Should I be using another repository or branch to get the latest?

I'll be available tomorrow, probably at home because of the weather forecast. 

Thanks,

Margaret