Revert "docs: disable PDF output for documentation generation"

This reverts commit 279e28ed35.

Reason for revert: Re-enable the pdf generation since the necessary fix to generate pdf has already been merged.

Change-Id: Ia68e8caea94b5d89fcf182251a13473b32b4bfad

.readthedocs.yaml

@@ -24,3 +24,8 @@ build:
 
 sphinx:
   configuration: docs/conf.py
+
+# Auxiliary formats to export to (in addition to the default HTML output).
+formats:
+  - pdf
+