Allow nested templates for root (#13)

When rendering jinja templates for the root directory and writing them to the build destination, a `FileNotFoundError` could be encountered. This is because the code only ensured the existance of the output directory but not of any subfolders templates might be placed into. This PR fixes this and creates the missing subdirectories when necessary.

Author: real-yfprojects

sphinx_polyversion/driver.py

@@ -678,4 +678,5 @@ async def build_root(self) -> None:
                 template = env.get_template(template_path_str)
                 rendered = template.render(context)
                 output_path = self.output_dir / template_path_str
+                output_path.parent.mkdir(parents=True, exist_ok=True)
                 output_path.write_text(rendered)