Add 'what is searched' message to cloud homepage

Created by: rrhyne

Add messaging to the home page to help new users understand what Sourcegraph is and what it searches.

Potential message:

Code search for the GitHub top 10k repositories or any Gitlab or Github repository you specify.

Design comps:

https://www.figma.com/file/1fySTh02CO50wsxva9dws3/What-we-search-messaging?node-id=1%3A1825&viewport=1897%2C785%2C1.287500023841858