API documentation fixes

Fixes:
1. `GET /api/agency/` does not display processing time or reading rooms so I fixed the example and moved the stats documentation to the `GET /api/agency/{{slug}}` section
2. Updated with Example API Call and results that reflect the current API
3. added "working" days to stats documentation. 

Should fix #333 and #366 

See how previously we had some fake FOIA library links: 
-    "foia_libraries": [{"url": "http://trains.gov/foia/library", "link_text": "FOIA Library"}],

We probably need to add those in, or pick an example that has everything filled in. Having an empty foia_libraries is not useful to users of the documentation.