Remove developer sections that we don't need

- Set Up Your Environment

- Develop from Source
Add note about issues in existing code
Improve guidelines for code review
Rename "Coding Guidelines" to "Style Guide"
Change ToC depth from 2 to 1 to hide details
Use better naming for titles and file names
Use more general description for pull requests
Fix link to styleguide
Write style guide page
add plugins tutorial
Merge branch 'add-plugin-tutorial-docs' of github.com:riolowry/freeseer-docs into add-plugin-tutorial-docs