Update .readthedocs.yml config to disable PDF and EPUB builds

(cherry picked from commit 122f3c7b62)

.readthedocs.yml

@@ -1,12 +1,27 @@
-# Reference: https://docs.readthedocs.io/en/latest/yaml-config.html
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
 
 build:
   image: latest
 
+sphinx:
+  configuration: conf.py
+
 # 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 epub and pdf for now to speed up builds
-formats:
-  - pdf
-  - 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: []
+
+python:
+  install:
+    - requirements: requirements.txt