[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.

Start by reading the current contributing documentation and Beni's linked blog post about creating custom framework integrations. Map the requested code tour, inheritance paths, and integration instructions into a single contributor-facing document. Done means contributors can find the main code areas, understand the relevant inheritance, and follow the documented integration guidance.

Written by the indexing model from the issue text.