Disable PDF generation on ReadTheDocs (#442)

### What kind of change does this PR introduce?

* Disable PDF generation

### Does this PR introduce a breaking change?

No.

### Other information:

The `latest` and `stable` builds of the documentation will try to build
a PDF using LaTeX, and unfortunately it isn't clear why this is failing.
There's likely a workaround that we can port from `xscen` or `xclim` to
get it working again, but, in the meantime, we want the docs to be
web-accessible at the very least.

.readthedocs.yml

@@ -22,8 +22,8 @@ build:
       - sphinx-build -M gettext docs docs/_build
       - sphinx-build -b linkcheck docs/ _build/linkcheck || true
 
-formats:
-  - pdf
+# FIXME: PDF builds not working since addition of the RavenPy logo
+formats: [ ]
 
 conda:
   environment: environment-docs.yml