Add doctests, docstrings, and docs

[gilch]

We might need a REPL first. #2. Lissp has doctests.

[gilch]

A tutorial in the style of learn X in Y minutes might also be good. Especially if we can doctest a REPL session.

I also really like the organization of https://clojure.org/api/cheatsheet, but Hebigo's API isn't that big. Sphinx autodoc like Hissp has may suffice.

An interactive Jupyter notebook tutorial is another possibility, but I'm not sure how it could be hosted. I've heard of free services, but I'm not sure if they would support our kernel. Pyodide might be an alternative. It might be powerful enough to run a Hebigo compiler.