[Docs] Better contributing documentation

Ref #681

Our contributing documentation is a bit lacking. Specifically, it would be nice to have a document that gives a tour of where the various pieces of the code are, how some of the inheritance paths work, and maybe pull some of the instructions from [Beni's old blog post](https://www.elastic.co/blog/creating-custom-framework-integrations-with-the-elastic-apm-python-agent) into our documentation proper.