Improve the contributing documentation

What does this MR do?

Improve the contributing documentation

This removes duplicated content from CONTRIBUTING.md and consolidate
everything under doc/development/contributing/.

This also fixes missing links.

Lastly, this moves the style guides from the "Implement design & UI
elements" page to a new "Style guides" page.

TODO:

• Checks that all links are working.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides