Build: fail builds without configuration file or using v1

Use a feature flag to decide whether or not hard fail the builds that are not
using a configuration file at all or are using v1.

This will be useful in the future when we want to make the builds to fail during
a reduced period of time to inform users/customers about this deprecation.

Related #10351

readthedocs/doc_builder/director.py

@@ -238,6 +238,12 @@ def checkout(self):
         self.data.build["config"] = self.data.config.as_dict()
         self.data.build["readthedocs_yaml_path"] = custom_config_file
 
+        # Raise a build error if the project is not using a config file or using v1
+        if self.data.project.has_feature(
+            Feature.NO_CONFIG_FILE_DEPRECATED
+        ) and self.data.config.version not in ("2", 2):
+            raise BuildUserError(BuildUserError.NO_CONFIG_FILE_DEPRECATED)
+
         if self.vcs_repository.supports_submodules:
             self.vcs_repository.update_submodules(self.data.config)
 

readthedocs/doc_builder/exceptions.py

@@ -57,6 +57,11 @@ class BuildUserError(BuildBaseException):
         "Ensure your project is configured to use the output path "
         "'$READTHEDOCS_OUTPUT/html' instead."
     )
+    NO_CONFIG_FILE_DEPRECATED = gettext_noop(
+        "Not using a '.readthedocs.yaml' configuration file is deprecated. "
+        "Add a configuration file to your project to make it build successfully. "
+        "Read more at https://docs.readthedocs.io/en/stable/config-file/v2.html"
+    )
 
 
 class BuildUserSkip(BuildUserError):

readthedocs/projects/models.py

@@ -1915,6 +1915,7 @@ def add_features(sender, **kwargs):
     DONT_CREATE_INDEX = "dont_create_index"
     USE_RCLONE = "use_rclone"
     HOSTING_INTEGRATIONS = "hosting_integrations"
+    NO_CONFIG_FILE_DEPRECATED = "no_config_file"
 
     FEATURES = (
         (ALLOW_DEPRECATED_WEBHOOKS, _("Webhook: Allow deprecated webhook views")),
@@ -2111,6 +2112,10 @@ def add_features(sender, **kwargs):
                 "Proxito: Inject 'readthedocs-client.js' as <script> HTML tag in responses."
             ),
         ),
+        (
+            NO_CONFIG_FILE_DEPRECATED,
+            _("Build: Building without a configuration file is deprecated."),
+        ),
     )
 
     FEATURES = sorted(FEATURES, key=lambda l: l[1])