Collate information on contributing to the project into a single source of truth.

As noted in #2333, information on how to contribute to the project is split over
- the wiki ['Contributing'](https://github.com/gchq/CyberChef/wiki/Contributing) page
- the main project [README.md](https://github.com/gchq/CyberChef/blob/master/README.md#contributing)
- the main project [CONTRIBUTING.md](https://github.com/gchq/CyberChef/blob/master/CONTRIBUTING.md)

This leads to duplication of information and updates getting out of sync.

The information should be collated into a single place and the other locations either removed or changed to a simple link to the main source of truth.

Compare the wiki Contributing page, the contributing section of README.md, and CONTRIBUTING.md to identify duplicated and conflicting information. Consolidate the guidance into one source of truth and replace the other locations with links; done means contributors can find the same current instructions without duplication.

Written by the indexing model from the issue text.