Add API to documentation

[MaxGhenis]

Fixes #114

I started by modeling after PSLmodels/Tax-Calculator#2441, testing with a single page (agg.py). However, jb build . throws the following error:

(base) mghenis@penguin:~/PSLmodels/microdf/docs$ jb build .
Running Jupyter-Book v0.8.0
Traceback (most recent call last):
  File "/home/mghenis/anaconda3/bin/jb", line 8, in <module>
    sys.exit(main())
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/jupyter_book/commands/__init__.py", line 199, in build
    build_modified = max([os.stat(ii).st_mtime for ii in build_files])
ValueError: max() arg is an empty sequence

@jdebacker could you please take a look to see if I got anything obviously wrong here, or could you recommend documentation on making this work? I didn't see this error in the executable-books org. Maybe microdf.agg isn't a proper module?

[MaxGhenis]

Huh, odd that jb built successfully via the GH Action.