Add docs summarizing which iNaturalist API endpoints pyinaturalist implements

[JWCook]

It would be useful to show in a concise manner which API endpoints pyinaturalist implements, so a new user can more easily get an idea of the features offered. For that purpose, I'd like to add a table listing all iNaturalist API endpoints, indicating which ones have been implemented in pyinaturalist.

Also, now that the list of API functions has grown, as well as the amount of documentation per function, the docs for the node_api and rest_api modules are now quite long. I'd like to add function summary sections at the top of each of these modules. There are a number of Sphinx extensions that could accomplish this.