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 #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 #4007

Checklist

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

Type of change

• [x] Documentation (new example notebook)