[Serve] [Doc] Mock ray.serve.generated package for doc building
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 756170923f8ff..05cc18898b7dc 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -61,6 +61,8 @@ def __getattr__(cls, name):
     "ray.core.generated.common_pb2",
     "ray.core.generated.gcs_pb2",
     "ray.core.generated.ray.protocol.Task",
+    "ray.serve.generated",
+    "ray.serve.generated.serve_pb2",
     "scipy.signal",
     "scipy.stats",
     "setproctitle",

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## Why are these changes needed?
Serve's API doc failed to build because it can't import from protobuf in readthedocs environment. We used the a common pattern to mock it out.

```
WARNING: autodoc: failed to import function 'serve.get_replica_context' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import function 'serve.start' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import function 'serve.deployment' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import function 'serve.list_deployments' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import function 'serve.get_deployment' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import function 'serve.shutdown' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import class 'serve.api.Deployment' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import class 'serve.handle.RayServeHandle' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
WARNING: autodoc: failed to import function 'serve.batch' from module 'ray'; the following exception was raised:
No module named 'ray.serve.generated'
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [  0%] _help
writing output... [  0%] actors
```


<!-- Please give a short summary of the change and the problem this solves. -->

## Related issue number
Fixes #18684
<!-- For example: "Closes #1234" -->

## Checks

- [ ] I've run `scripts/format.sh` to lint the changes in this PR.
- [ ] I've included any doc changes needed for https://docs.ray.io/en/master/.
- [ ] I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
- Testing Strategy
   - [ ] Unit tests
   - [ ] Release tests
   - [ ] This PR is not tested :(
