[FEATURE] Add Sphinx documentation

I already got started on a [branch ](https://github.com/omar2535/GraphQLer/tree/sphinx) that contains some boilerplate to generate [spinx](https://www.sphinx-doc.org/en/master/) documentation. Since GraphQLer has a lot of handy functions, it would be useful to have a [read-the-docs](https://docs.readthedocs.io/en/stable/) for GraphQLer built-ins.