Convert docs to read the docs

[catanzaromj]

This PR introduces a read the docs documentation build rather than github pages. If this is desired, you can remove the github pages build in a future PR once everything is working and in place. The docs based off this PR can be viewed here if you have access to scikit-tda on RTD. If you don't, well they look great! All of the notebooks are compiling and the API links working. If you pass along your RTD username, I can add you to the RTD admin page and you can view it yourself.

I added a docs section to setup.py rather than a docs/requirements.txt file. I prefer this style, since it's easer for a user to install these additional requirements when they install the package (if they desire). If you prefer the latter, I can switch it over.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.61%. Comparing base (4215778) to head (a59fa87).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #16 +/- ## ======================================= Coverage 72.61% 72.61% ======================================= Files 10 10 Lines 1048 1048 Branches 189 189 ======================================= Hits 761 761 Misses 268 268 Partials 19 19 ```

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[catanzaromj]

If this all seems good, then we can host the docs at dreimac.scikit-tda.org if you'd like, in line with the other scikit-tda projects. We can also link to DREiMac from the main scikit-tda docs as well.