Static checks for html documentation

This commit introduces the static checks for avocado documentation. This
change will help us avoid documentation issues in the future. It makes
the readthedocs build more strict, which make it fail on any warning.

Reference: avocado-framework#5757
Signed-off-by: Jan Richter <[email protected]>

.readthedocs.yaml

@@ -14,6 +14,7 @@ build:
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/source/conf.py
+  fail_on_warning: true
 
 # We recommend specifying your dependencies to enable reproducible builds:
 # https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html