Clean up the Developer section

- [x] Remove sections that aren't useful
- [x] Rename headers and filenames to be more descriptive
- [x] Change ToC depth from 2 to 1 (don't need to show the details from subsections)
- [x] Improve guidelines for code review
- [x] Write styleguide page (only contains todo's right now)
- [ ] ~~Refine testing page (e.g. concise titles, neat formatting, readability, typos, and so on)~~