Docs: Repo Landing Page Experience

[astorm]

It's come up a few times elsewhere -- the landing page page experience of https://github.com/elastic/apm-aws-lambda could use some work. Problems to solve

1. Empty README.md in the main repository
2. Visibility of docs folder for lambda extension
3. Should the extension remain in its own sub-folder in anticipation of the repository containing other sub-projects (layers for automatically deploying the agent(s), etc.), or should the extension be the top level project?

[astorm]

With #76 landed this is, for now, done. Closing.