Contributing guidelines overhaul

[adamrtalbot]

Description of feature

After opening a recent pull request, I found the contributing guidelines to be quite scattered. I think all of the content is there, but it's spread out and not clear where I should be looking.

We could clean up the contributing guidelines into a central place with a clear flow to follow. An experienced developer would follow the checklist, a newbie could follow it as a tutorial.

I think Mozilla has a nice overview page of what a contributing guideline should contain: https://mozillascience.github.io/working-open-workshop/contributing/.

Links:

• Guide for writing a module: https://multiqc.info/docs/development/modules/
• Contributing page on docs website: https://multiqc.info/docs/development/contributing/
• CONTRIBUTING.md: https://github.com/MultiQC/MultiQC/blob/main/.github/CONTRIBUTING.md