Add runner 2 and 3 documentations (#488)

Reviewed-on: https://gitea.com/gitea/docs/pulls/488
Reviewed-by: silverwind <[email protected]>

runner-docs_versioned_sidebars/version-0-sidebars.json

@@ -0,0 +1,9 @@
+{
+  "runner": [
+    {
+      "type": "doc",
+      "id": "index",
+      "label": "Gitea Runner"
+    }
+  ]
+}