docs: cross-link AutoML and zero-shot LightGBM notebooks

[Arushi Sagar (arushiiii18)]

Why are these changes needed?

Right now the AutoML tuning notebook (automl_lightgbm.ipynb) and the zero-shot notebook (zeroshot_lightgbm.ipynb) don't reference each other. Someone who opens one has no way of knowing the other approach exists, even though they solve the same problem differently (tuning vs. instant data-dependent defaults). Added a short note in each notebook, right before the install step, linking to the other so users can pick the approach that fits their use case before running anything.

Related issue number

Closes #737

Checks

• I've used pre-commit to lint the changes in this PR (note the same in integrated in our CI checks).
• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[Arushi Sagar (arushiiii18)]

@microsoft-github-policy-service agree

[Li Jiang (thinkall)]

Thank you very much for the PR, Arushi Sagar (@arushiiii18) !