Build: fail builds if there is no index.html in the output dir

We announced we deprecated this in readthedocs/blog#230
and we already contacted the users to warn about this.

This PR has to be merged after July 25th, 2023 and it has to be deployed on
August 1st.

Closes #1800

Author: humitos

readthedocs/projects/tasks/builds.py

@@ -574,24 +574,13 @@ def get_valid_artifact_types(self):
 
             if artifact_type == "html":
                 index_html_filepath = os.path.join(artifact_directory, "index.html")
-                readme_html_filepath = os.path.join(artifact_directory, "README.html")
-                if not os.path.exists(index_html_filepath) and not os.path.exists(
-                    readme_html_filepath
-                ):
-                    log.warning(
+                if not os.path.exists(index_html_filepath):
+                    log.info(
                         "Failing the build. "
                         "HTML output does not contain an 'index.html' at its root directory.",
                         index_html=index_html_filepath,
-                        readme_html=readme_html_filepath,
                     )
-                    # TODO: uncomment this line to fail the build once we have
-                    # communicated with projects without an index.html or
-                    # README.html
-                    #
-                    # NOTE: we want to deprecate serving README.html as an
-                    # index.html file as well.
-                    #
-                    # raise BuildUserError(BuildUserError.BUILD_OUTPUT_HTML_NO_INDEX_FILE)
+                    raise BuildUserError(BuildUserError.BUILD_OUTPUT_HTML_NO_INDEX_FILE)
 
             if not os.path.exists(artifact_directory):
                 # There is no output directory.