Create documentation on ReadTheDocs

The two blog posts that motivated this library contain some useful references for documentation:

https://ilovesymposia.com/2017/03/12/scipys-new-lowlevelcallable-is-a-game-changer/

https://ilovesymposia.com/2017/03/15/prettier-lowlevelcallables-with-numba-jit-and-decorators/

Currently, llc-tools doesn't have any documentation. It would be good to put some on ReadTheDocs or GitHub Pages.

Other useful references:

https://docs.scipy.org/doc/scipy/reference/generated/scipy.LowLevelCallable.from_cython.html

https://docs.scipy.org/doc/scipy/reference/ccallback.html

Start by reading the two linked blog posts and the SciPy LowLevelCallable references to identify the useful documentation topics for llc-tools. Done means publishing documentation for the library on ReadTheDocs or GitHub Pages, with the scope and hosting choice resolved.

Written by the indexing model from the issue text.