[Docs] Fix swagger rendering in REST API docs

- Add generated openapi.json to source directory so render_swagger can find it
- Enable allow_arbitrary_locations in render_swagger config
- Gen-files plugin will keep the file updated during builds

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: peterschmidt85

docs/docs/reference/api/rest/openapi.json

@@ -148,7 +148,8 @@ plugins:
           options:
             docstring_style: google
             show_source: false
-  - render_swagger
+  - render_swagger:
+      allow_arbitrary_locations: true
 
 # Extensions
 markdown_extensions: