Use GitHub's Open Source Guides to explain how to contribute #5

GitHub recently announced a new Open Source Guides website that is

a collection of resources for individuals, communities, and companies who want to learn how to run and contribute to open source.

We might want to use these Open Source Guides for our docs to help people contributing to Pico. This needs to be evaluated though, I just skimmed through the contents.

https://github.com/blog/2318-announcing-open-source-guides
https://opensource.guide/