Sphinx setup

Closes #104 (closed)

This adds a setup that is very close to Ponos': recommonmark is available to write docs in both Markdown and reStructuredText and the readthedocs theme has been used. The documentation documentation (docs/dev/build_docs.md) has been almost completely copy-pasted from Ponos, and autosummary has been used to get all of arkindex_worker included in the documentation immediately; we can work on cleaning that up later, maybe with a bit less automation—Ponos does not use autosummary to instead get cleaner docs.