New deploy docs

Created by: beyang

A new index page for Sourcegraph deployment documentation.

To view in local dev,

1. Check out this branch
2. Run yarn docsite:serve from the repo root
3. Point your browser at http://localhost:5080/next

4.0 launch TODOs

• One-click AWS AMI docs, modeled after how Retool does it in their self-hosted portal on my.retool.com (Stephen)
  • Instance sizing recommendations for one-click AMI (James)
• Make HTML and CSS on the index page production-quality (Beatrix)
• Update links from homepage and main docs page (Beyang)

Stretch

• Revise and streamline other docs linked from this page