docs: simplify contributing page

[fyliu]

Overview

We need contributing docs more suitable for contributors of our project that should already know the basics. The main page should be kept simple with links to more detailed explanations. A short but useful contributor intro to the project will make it more accessible.

Action Items

• [ ] Create a contributing folder in docs/ to hold explanation pages
• [ ] Move most or all sections out of the contributing page into pages in contributing/
• [ ] Replace sections with short descriptions and links

Instructions

• Keep the contributing page short but usable by experienced devs
• Link to detailed pages for clarification

[fyliu]

Edited the issue description around the goal of simplifying the page. Removed text that should go into separate issues.