Add sphinx documentation of API functionality

[corviday]

Adds sphinx for API documentation.

Because we prefer Markdown to reStructured Text, almost all text is written in markdown files that are then included into the rST files with m2r, and sphinx's native rST files are used for structure only. The exceptions are the main index file, and the help for the metadata endpoint, which takes advantage of functionality only available in rST to put a big red warning box on the page.

There are no real changes to any files outside the newly created folder /doc, just whitespace formatting on docstrings of API endpoint functions to make them display better in sphinx, and transferring a docstring to a different function in watershed.py.

[corviday]

@nikola-rados , could you look over this when you have time?

• Try downloading the branch and see if the documentation builds correctly (instructions have been added to the README).

• If documentation builds correctly, it will show up in doc/build/html. Read the documentation and see if it all seems correct, generally makes sense.

• Let me know if you think of anything else I should cover?

[nikola-rados]

The docs were successfully built. Looking through them I think they are comprehensive and no further additions are required. Great work!

[corviday]

• add to the documentation documentation that you need to install latexmk to generate a PDF