Revert "Disable epub and PDF builds - no one needs them, and they mak…

…e the build 3x slower" (#322)

This reverts commit fae2781.

ReadTheDocs is now configured to use a single sphinx build to generate all three outputs.
As a result, the notebooks should now be run just once.

.readthedocs.yml

@@ -10,6 +10,9 @@ sphinx:
   configuration: docs/conf.py
   fail_on_warning: false
 
+# Optionally build your docs in additional formats such as PDF and ePub
+formats: all
+
 # Optionally set the version of Python and requirements required to build your docs
 python:
   version: 3.8