Add example demonstrating many-to-many relationship using association…

[Vedavyasa21]

This PR adds a simple example that demonstrates how to represent many-to-many relationships in Featuretools using an association (bridge) entity.

Details

• Added new example: docs/source/examples/many_to_many_relationship.py
• Shows how two one-to-many relationships (students → enrollments, courses → enrollments) can model a many-to-many relationship.
• Clarifies how users can structure data for Deep Feature Synthesis in such cases.

Motivation

Addresses #54 and helps clarify the concept of "association entities" for new users.

Checklist

• Example runs successfully
• Follows existing demo structure
• References relevant issue (Provide example of many-to-many relationship #54)

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[Vedavyasa21]

Hey! @alteryx Once check the PR