Revert "docs: disable PDF output for documentation generation"

This reverts commit 279e28ed3526bfcfb8d8bc736e4f5aafd3d73ba3.

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

Change-Id: Ia68e8caea94b5d89fcf182251a13473b32b4bfad
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 6207066..e3a7ebf 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -24,3 +24,8 @@
 
 sphinx:
   configuration: docs/conf.py
+
+# Auxiliary formats to export to (in addition to the default HTML output).
+formats:
+  - pdf
+