Add core notebook demonstrating vectorized/shaped variables (Fixes #4007)

[Abhishek6122008]

Description

This PR adds a new core notebook vectorized_variables.ipynb under
docs/source/learn/core_notebooks/.

The notebook provides a clear and concise tutorial demonstrating how to work
with vectorized and shaped random variables in PyMC. It addresses the request
in Issue #4007 by showing:

• How to define RVs with shapes / dims / coords
• How to slice and index vector-valued parameters (e.g., mu[data_labels])
• How broadcasting works in PyMC models
• How to model multiple groups simultaneously using vectorized priors
• A small end-to-end example model that mirrors the use case described in Documentation: Create a tutorial demonstrating vector variables #4007

The goal is to give users a direct, accessible reference for one of the most
common modeling patterns in PyMC: handling multiple groups with vector-valued
parameters.

Related Issue

• Closes Documentation: Create a tutorial demonstrating vector variables #4007

Checklist

• Added new example notebook
• Confirmed the notebook renders correctly in the docs
• Checked pre-commit linting
• Tests not required for documentation notebooks

Type of change

• Documentation (new example notebook)

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