Improve contributing docs

While helping onboard a new contributor a few issues encountered were:

* "Deploying" is confusing when we are an infrastructure project. Thought this meant "deploying to google cloud" when it means "Releasing a new version of the backend"
* Also thought that she was meant to deploy an infrastructure to contribute. Need to make this much more clear that it is entirely possible (and encouraged) to contribute to CDP without needing to deploy a dev infrastructure