Unify the two contributing docs

- https://github.com/INCATools/ontology-access-kit/blob/main/.github/CONTRIBUTING.md
- this focuses entirely on code style
- https://incatools.github.io/ontology-access-kit/contributing.html
- source: https://github.com/INCATools/ontology-access-kit/blob/main/docs/contributing.md
- this is a bit more detailed

I like having contributor guide be integrated into the main docs, this will likely grow; I also like having a CONTRIBUTING.md in a standard place

I am leaning towards having CONTRIBUTING.md be minimal, and having both docs hyperlink one another. Perhaps the one that is integrated into the docs be more of a "core developers guide"

Sometimes I have seen the same doc used, with a copy or symlink between the two folders, but, I am not a fan of this.

Compare .github/CONTRIBUTING.md with docs/contributing.md, which is rendered as the contributing guide, and review how each currently presents contributor information. Resolve the preferred relationship between the two documents and make the result consistent, including the proposed cross-links. Done means both standard and integrated documentation locations clearly point contributors to the right guide without duplicated or conflicting instructions.

Written by the indexing model from the issue text.