Refactor notebook generation

[leloykun]

Context

• preprocess.py and build_tex are borderline unreadable for newbies

TODOs

• Remove unused codes & constants (such as TO_HASH & all the hash-related stuff in preprocess.py
• Convert code directory, column width, and other constants into file arguments
• Wrap the codes into main functions and use if __name__ == "__main__"
• Make the codes more modular. Ideally each subfunction should only be < 60 lines long
• Document each subfunction