Disable PDF generation again

It's still broken; it produces mangled characters like the following:

> attrÂaIDÂaid = C_EXP("PDQID_$id$");

.readthedocs.yaml

@@ -10,7 +10,7 @@ sphinx:
    configuration: docs/conf.py
 
 # Optionally build your docs in additional formats such as PDF
-formats: [htmlzip, pdf]
+formats: [htmlzip]
 
 # Optionally set the version of Python and requirements required to build your docs
 python: