Update CONTRIBUTING.md to point to actual contributing guide in docs

[canyon289]

Description of your problem

I'm trying to contribute docs but I'm not sure how to compile them. Contributing guide doesn't tell me how to go about this, just how to push changes, and mostly focuses on code changes

https://github.com/pymc-devs/pymc/blob/main/CONTRIBUTING.md

See @OriolAbril's comment below

[OriolAbril]

We do at https://docs.pymc.io/en/latest/contributing/build_docs.html. We should update the github contributing thing to be only a link to the actual contributing guide in the docs