Use Doxygen for generating documentation

• [x] Add Doxyfile
• [x] Update comments using the Doxygen style
• [ ] Use the README as the documentation's main page
• [ ] Update Doxyfile to be more compatible with a single class project like BigInt (remove class list and a few other nav sections (to be decided))
• [ ] Create a setup for the generated docs to be published via GitHub Pages

Closes #4.