Docs: Add vignette for tuning `fit_*` and `compile_*` arguments

[davidrsch]

What part of the documentation is this about?

The vignettes section of the documentation.

What is the issue?

The documentation lacks a clear, comprehensive example of how to tune important training parameters like epochs, batch_size, optimizer, loss, and learn_rate. This may make it difficult for users to understand how to optimize these crucial aspects of the Keras training process within the tidymodels framework.

Suggested improvement

A new vignette should be created to provide a detailed, step-by-step tutorial on how to tune these fit_* and compile_* arguments. This guide should demonstrate how to:

• Define a kerasnip model specification with tunable fit_* and compile_* arguments.
• Use dials and kerasnip's special parameter functions (optimizer_function(), loss_function_keras()) to create a hyperparameter grid for these arguments.
• Run a standard tidymodels tuning workflow to find the optimal training parameters.
• Finalize the workflow and fit the model with the best parameters.

Additional context

Adding this guide would fill a significant gap in the documentation and provide users with a much-needed, reproducible example for a common and important task.