Merge pull request gitpython-developers#1856 from EliahKagan/doc-dl

Restore building of documentation downloads

.readthedocs.yaml

@@ -23,9 +23,7 @@ sphinx:
   fail_on_warning: true
 
 # Optionally build your docs in additional formats such as PDF and ePub.
-# formats:
-# - pdf
-# - epub
+formats: all
 
 # Optional but recommended, declare the Python requirements required
 # to build your documentation.