Reorganize the getting started sections

[mrobinson]

The current organization of the book is a bit hard to follow as it mixes running the nightly version of servoshell with getting started working on Servo. I'd like to suggest the following reorganization of the content.

1. Trying Servo

• Downloading the nightly
• Running the nightly

2. Building Servo

• Fetch the source
• Set up your environment
• Building
• Troubleshooting build issues

3. Contributing

• Git workflow
• Creating pull requests
• the rest of the Hacking on Servo section
• Contributing to the book

4. Architecture

• ...

Then we would remove "Docs for older versions" and "Pre-book docs" or put them on a single page as they are interesting, but shouldn't distract from current contents.