Mkdocs: default to "docs" for docs_dir

This value has been the default since the start of the project
https://github.com/mkdocs/mkdocs/blob/0.13.0/mkdocs/config/defaults.py#L44

We should respect that default value,
(we should actually don't edit it at all #2483).

So, instead of trying to guess the docs dir always,
only try to guess it if the user doesn't have mkdocs.yaml file.

This should avoid any backwards compatibility and avoid weird bugs to
new users.

Closes #7539

readthedocs/doc_builder/backends/mkdocs.py

@@ -115,6 +115,7 @@ def load_yaml_config(self):
             )
             return {
                 'site_name': self.version.project.name,
+                'docs_dir': self.docs_dir(),
             }
         except yaml.YAMLError as exc:
             note = ''
@@ -139,14 +140,12 @@ def append_conf(self):
         user_config = self.load_yaml_config()
 
         # Handle custom docs dirs
-        user_docs_dir = user_config.get('docs_dir')
-        if not isinstance(user_docs_dir, (type(None), str)):
+        docs_dir = user_config.get('docs_dir', 'docs')
+        if not isinstance(docs_dir, (type(None), str)):
             raise MkDocsYAMLParseError(
                 MkDocsYAMLParseError.INVALID_DOCS_DIR_CONFIG,
             )
 
-        docs_dir = self.docs_dir(docs_dir=user_docs_dir)
-
         self.create_index(extension='md')
         user_config['docs_dir'] = docs_dir