Starting from Nuxeo Platform 5.9.3, the Nuxeo API Playground (currently in beta version) offers a Resource Endpoints section to discover all the endpoints. We strongly encourage you to browse this section for a better understanding and a description for expected URLs, parameters, headers, etc. Note though that this section has just been added and is still in beta version, some evolutions will come regularly on this sandbox. You may also want to check out the reference list of existing JSON entity types.
You can also find some complementary endpoints documentation on the server itself, at the URL nuxeo/api/v1/doc. Beware that endpoints that are onnuxeo/api/v1/doc and not on Nuxeo API Playground are subject to some evolutions before 6.0 (especially the web adapters).