WIP: Getting started guides: volume 1

Created by: ryan-blunden

Adds a Getting started section to the docs as per RFC 119

Most of these pages are still a work in progress.

To preview, view the getting started index page, then use the next links at the bottom of each page to navigate through, or use the list below:

• Getting started
• Choosing the right deployment model
• Installing Sourcegraph
• Integrating your code host
• Universal Code Search
• Universal Code Intelligence
• Next steps