DOC Coalesce README.rst and index.rst

[jacksonllee]

This PR merges the repo's top-level README.rst and the Sphinx docs-specifics index.rst. The end result is that README.rst is the single source of truth for such items as installation and API keys, whereas index.rst is now minimal and contains (i) markers that draw text from README.rst and (ii) items specific to Sphinx docs such as toctree.

For the new README.rst, please check it out at my fork: https://github.com/jacksonllee/civis-python/blob/coalesce-readme-index-rst/README.rst

For the new index.rst, the compiled Sphinx docs will look like this:

Resolves #253.