Disable additional docs formats

Disable the addition HTMLZip, PDF and Epub documentation formats in the
readthedocs build. These are causing warnings that're causing the build
to fail, and they don't work anyway: various things like the table of
contents, API docs, etc don't work in the additional formats.

.readthedocs.yaml

@@ -7,7 +7,7 @@ sphinx:
   configuration: docs/conf.py
   builder: "dirhtml"
   fail_on_warning: true
-formats: all
+formats: []
 python:
   install:
     - requirements: requirements/docs.txt