docs(lib): library overview as far as possible

Everything we have, feature wise, is now documented in a first version
at least.

We shall keep this uptodate with what we are implementing, which also
helps figuring out a good api.

etc/api/shared.yaml

@@ -13,6 +13,9 @@ api:
     - name: youtube
       version: v3
   base_path: "etc/api"
+  terms:
+    # how to actually do something with the API
+    action: do
   templates:
   # all output directories are relative to the one set for the respective API
     - source: README.md