Add OpenApi spec and Swagger documentation

Add Swagger documentation to the root
http://127.0.0.1:1969/?doc with OpenApi 3.0
specification at http://127.0.0.1:1969/?spec

Addresses zotero#76

Bug: T291707
Change-Id: Ide7b45e7dca90b3ccbbf8141358a66f8dc7b1187
(cherry picked from commit 42bc4af)

Author: mvolz

README.md

@@ -79,6 +79,10 @@ It’s also possible to opt out of proxying for specific hosts by using the `NO_
 
 ## Endpoints
 
+### Documentation
+
+Swagger documentation is available at http://127.0.0.1:1969/?doc
+
 ### Web Translation
 
 #### Retrieve metadata for a webpage: