docs: Disable additional formats on readthedocs.org

The additonal formats increase build times and right now fail with
current furo.

See readthedocs/readthedocs.org#3220
See pradyunsg/furo#710

.readthedocs.yml

@@ -9,7 +9,7 @@ sphinx:
   configuration: docs/conf.py
 
 # Optionally build your docs in additional formats such as PDF and ePub
-formats: all
+formats: []
 
 build:
   os: ubuntu-22.04