Single source of truth for contributing documentation

### Describe the feature you'd like

At the moment Wave has two different places where contribution guidelines are documented:

- [Wave's documentation site](https://wave.free-now.com/contributing)
- [CONTRIBUTING.md](https://github.com/freenowtech/wave/blob/main/CONTRIBUTING.md)

This can lead to potential contributors not having the full picture believing that they've already read the full contribution guidelines when in reality they've only read one of the two parts (it happened to me, I only read the CONTRIBUTING.md and after 6 months in the company I realised there was more...😅).

The idea would be to unify these two sources into one, we could only have the contribution guidelines in the documentation site and then a CONTRIBUTING.md that simply points to the site. This is the same approach that React uses, check out their [CONTRIBUTING.md](https://github.com/facebook/react/blob/main/CONTRIBUTING.md) (it simply points to their [documentation site](https://reactjs.org/docs/how-to-contribute.html)).

I believe this would be pretty straightforward and it will make the DS easier to contribute to for new people, I can create a PR for this unification myself if enough people agree.

Compare the contribution guidelines on the documentation site with CONTRIBUTING.md, using the linked Wave pages as the starting points. Consolidate the guidance into one source and make CONTRIBUTING.md point to it; done means contributors can reach the complete guidelines from either entry point without duplicated instructions.

Written by the indexing model from the issue text.