Reenable PDF and EPUB builds for stable doc

They were disabled on master as they make builds way too long,
but for the stable branch they make sense, as it is synced manually.

(cherry picked from commit 763f6a9)

.readthedocs.yml

@@ -7,4 +7,6 @@ build:
 # 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: []
+formats:
+  - pdf
+  - epub