docs: def find_project_config_file(project_root: str) -> str:
      """Return absolute path to project-specific config file, if it exists.
  
      :param project_root: Absolute path to project root directory.
  
      A project config file is a file named `YCONFIG_FILE` found at the top
      level of the project root dir.
  
      Return `None` if project root dir is not specified,
      or if no such file is found.
      """
      if project_root:
          project_config_file = os.path.join(project_root, YCONFIG_FILE)
          if os.path.isfile(project_config_file):
              return project_config_file