[UPDATE] Fix broken Gymnasium migration guide link

# What do you want to improve?

The link to the Gym vs Gymnasium migration guide is outdated and returns a 404 error.

Current link:

https://gymnasium.farama.org/content/migration-guide/

Expected link:

https://gymnasium.farama.org/introduction/migration_guide/

This link appears in the section explaining the differences between Gym and Gymnasium. Updating it would help learners access the correct documentation.

Start in the documentation section explaining the differences between Gym and Gymnasium and locate the outdated migration-guide URL. Replace it with the expected introduction/migration_guide/ link, then verify that the link resolves successfully.

Written by the indexing model from the issue text.