How do I access the documentation for the Signals Notebook API (i.e,. Swagger UI)?

Once you have logged into Signals Notebook, select Your name on the right-hand top corner, from the drop-down select 'System Configuration' --> enter Your credentials to log in to Signals NB as Administrator User.
Again, select Your name on the right-hand top corner, from the drop-down select 'System Configuration Guide' -->
On the right-hand top corner search for 'Swagger', it will return "External Notifications" and "Query Operators in Search APIs"

Also, once You have logged in as an Administrator User, select "System Settings"--> "API Key"--> Open API Document.