Add documentation using `doxygen`

[dfintha]

Since barge initialized the repository with a Doxyfile and can be used to generate documentation with it, the source code could be documented using doxygen comments. Even though it comes with a slight cost of visual noise, this is a learning project, so detailed in-code documentation is acceptable.

[dfintha]

The documentation shall explain each function's role, any the following.

• Tokenizing using a state machine.
• Recursive descent parsing.
• Peephole optimizations.