Document watershed APIs

[corviday]

Updates documentation around the streamflow APIs:

• adds documentation for the downstream and watershed_streams APIs (resolve #216)
• adds documention of the Melton ration provided by @AreliaTW to the watershed API (resolves #193)

@AreliaTW also provided images illustrating the Melton Ratio, but I am not sure how to include them. I shortened her text slightly for the purposes of an API document - API users don't need to know how the files are organized on the backend.

[corviday]

Here's a zip file containing the html output of the new docs. The ones that have been updated are API docs for watershed, downstream, and watershed_streams, if you'd rather look at the changes in HTML than in raw markdown.

new_docs.zip

[corviday]

Huh, the html looked fine on my workstation. I must have just left some necessary files out of the .zip.