README updates for the main repo, and a light overview on the docs subdir

[akleiner2]

Overview Updating some links now that the new evergreen.segment.com is live.

Screenshots (if applicable) N/A

Documentation

• [ ] ~Updated Typescript types and/or component PropTypes~
• [ ] ~Added / modified component docs~
• [ ] ~Added / modified Storybook stories~

[akleiner2]

Need to figure out why the actions keep getting cancelled, but given that this is a documentation change, it should be fine.