Build: log usage of old output directory _build/html

We have changed the output directory from `_build/html` to
`$READTHEDOCS_OUTPUT/html` in the last few weeks.

However, there are some projects that may have `_build/html` directory hardcoded
in different places (e.g. `conf.py`). We want to log these projects and contact
them to communicate this situation.

Ideally, we should show a warning in the build detail's page, but I don't think
we currently have a way to display a warning message. We only support error
messages in this page.

Related #10036

Author: humitos

readthedocs/telemetry/collectors.py

@@ -92,8 +92,27 @@ def collect(self):
             },
         }
         data["doctool"] = self._get_doctool()
+
+        # NOTE: Check if there are files at `_build/html` (the old output directory)
+        # and log these projects so we can communicate with their maintainers
+        #
+        # This temporal and should be removed in the future once we have
+        # decided what to do with this collected data.
+        self._check_using_old_output_directory()
+
         return data
 
+    def _check_using_old_output_directory(self):
+        code, stdout, stderr = self.run(
+            "ls",
+            "_build/html",
+            cwd=self.project.checkout_path(self.version.slug),
+        )
+        if code == 0:
+            log.warning(
+                "Directory '_build/html' exists. This may lead to unexpected behavior."
+            )
+
     def _get_doctool_name(self):
         if self.version.is_sphinx_type:
             return "sphinx"