Use category instead of place holder markdown file as directory (#299)

Reviewed-on: https://gitea.com/gitea/docs/pulls/299

runner-docs/index.mdx

@@ -0,0 +1,8 @@
+---
+sidebar_position: 1
+slug: /
+---
+
+import ActRunnerDoc from '@site/docs/usage/actions/act-runner.mdx';
+
+<ActRunnerDoc />