add analyzing models and custom training how-to guides

[jmoralez]

Description

Adds the following guides:

• Analyzing models: provides a way to inspect the model parameters and compute shap values.
• Custom training: describes how to leverage mlforecast to perform the feature engineering, then train the model in any way and return it back to mlforecast to compute the forecasts.

Checklist:

• [x] This PR has a meaningful title and a clear description.
• [x] The tests pass.
• [x] All linting tasks pass.
• [x] The notebooks are clean.

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