docs: Fix docs build by setting uv conflicts for cu12 vs cu13

[chtruong814]

What does this PR do ?

docs: Fix docs build by setting uv conflicts for cu12 vs cu13

The docs build is failing because uv cannot resolve the dependencies after these changes:
#15506

uv is trying to resolve the environment for both cu12 and cu13 extras, so we need to update the pyproject.toml to indicate that both cannot be installed at the same time. Currently, uv is only used for docs building right now. Longer-term we will likely move Speech to use uv for dependency management overall.

https://github.com/NVIDIA-NeMo/NeMo/actions/runs/23536022366/job/68511293341

Using CPython 3.12.3 interpreter at: /usr/bin/python3
Creating virtual environment at: .venv
Activate with: .venv/bin/activate
  × No solution found when resolving dependencies for split (markers:
  │ python_full_version >= '3.14' and sys_platform == 'win32'):
  ╰─▶ Because nemo-toolkit[cu13] depends on cuda-python{sys_platform
      != 'darwin'}>=13,<14 and nemo-toolkit[cu12] depends on
      cuda-python{sys_platform != 'darwin'}>=12.6.0,<13, we can conclude that
      nemo-toolkit[cu12] and nemo-toolkit[cu13] are incompatible.
      And because your project requires nemo-toolkit[cu12] and
      nemo-toolkit[cu13], we can conclude that your project's requirements
      are unsatisfiable.

      hint: While the active Python version is 3.12, the resolution failed for
      other Python versions supported by your project. Consider limiting your
      project's supported Python versions using `requires-python`.

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[chtruong814]

The uv.lock file is not used by CI right now. Moving forward with fast merging this.