RTD: Re-disable EPUB builds for stable branch

Nope, too big and seen as invalid by most readers. See #3862.
There's really no good option for offline downloads currently.

.readthedocs.yml

@@ -4,9 +4,13 @@ build:
   image: latest
 
 # Possible options: htmlzip, pdf, epub
-# Disable (single) htmlzip as it's way too big to be useful,
-# and leads to excessive memory usage on build
-# Disable pdf as it generates too many latex warnings and the build
-# is killed once logs reach 4 MB.
-formats:
-  - epub
+# All disabled for now:
+# - single-page htmlzip is too big to be usable, and requires too much memory.
+# - pdf generates too many latex warnings and the build is killed once logs
+#   reach 4 MB. Could likely be improved if someone is motivated.
+# - epub is too big, and has tons of validation errors which make most readers
+#   treat it as invalid (GH-3862). Also, it's ugly.
+# Hopefully one day we'll have a multi-page HTML zip option, but until
+# then, all offline download options are worthless.
+# (Track https://github.com/readthedocs/readthedocs.org/issues/3242)
+formats: []