Update Tango API documentation

[damianhxy]

There are several inadequacies with the current documentation:

• Should be made clear what key refers to - Tango reads the RESTFUL_KEY env var (or uses test as a default). For docker installs, it's set by the .env file
• It should then be made clear in all the request headers that key is a variable, not verbatim
• open endpoint: currently always returns a empty dict (see Return MD5 of existing files on open Tango#145 for context)
• Should be made clear what the value of courselab is. It seems to be of the format <course name>-<asmt name>
• poll endpoint: outputFile is a variable whose value is of the form <user email>_<ver>_<assessment name>_autograde.txt (uri-encoded)
• inconsistency: info and jobs titles have slashes while the endpoint titles don't
• inconsistency: info endpoint is the only one that uses <KEY> to indicate that key is a variable
• inconsistency: some endpoints use empty to indicate empty request bodies, other endpoints use { <empty> }
• inconsistency: jobs endpoint has extraneous autograde.me in URL
• jobs endpoint: should be made clear that deadjobs should be an integer, either 0 or 1. Also, it's a GET method, not POST method
• pool endpoint: missing description of request body (empty). Could also mention that omitting image would show all pools.