Raise RTD doc build warnings as errors

.readthedocs.yml

@@ -8,6 +8,7 @@ version: 2
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/source/conf.py
+  fail_on_warning: true
 
 # Build documentation with MkDocs
 #mkdocs:

docs/source/conf.py

@@ -419,3 +419,7 @@ def find_source():
 
 """
 coverage_skip_undoc_in_source = True
+
+suppress_warnings = [
+    'epub.unknown_project_files'
+]