Migrates ai/docbot to uv to manage dependencies

[wwwillchen]

Our build started breaking today because llama-index packages had a version bump which causes an exception and broke docbot:

Traceback (most recent call last):
  File "/home/runner/work/mesop/mesop/ai/docbot/docs_index.py", line 144, in <module>
    llm = Gemini(model="models/gemini-1.5-flash-latest")
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/llama_index/llms/gemini/base.py", line 149, in __init__
    supported_methods = self._model_meta.supported_generation_methods
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pydantic/main.py", line 807, in __getattr__
    return self.__pydantic_private__[item]  # type: ignore
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pydantic/main.py", line 825, in __getattr__
    return super().__getattribute__(item)  # Raises AttributeError if appropriate
AttributeError: 'Gemini' object has no attribute '__pydantic_private__'. Did you mean: '__pydantic_complete__'?
./ai/docbot/deploy_to_hf.sh: line 29: cd: ./ai/docbot: No such file or directory

https://docs.astral.sh/uv/ seems like a pretty good tool for managing dependencies and enforces a lock file to prevent inadvertent dependency upgrades like the one that happened here, to ensure reproducible build.

[wwwillchen]

@richard-to going to merge this to test if it works on the main branch, but let me know what you think about uv.