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

• [x] Example runs successfully
• [x] Follows existing demo structure
• [x] References relevant issue (#54)

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Vedavyasa21]

Hey! @alteryx Once check the PR