`mesa-3.x` legacy branch for Mesa 3.x examples

We've created a [`mesa-3.x`](https://github.com/projectmesa/mesa-examples/tree/mesa-3.x) branch for Mesa 3.x examples, while we continue development towards Mesa 4.0.

For new pull requests:
- To update/add an example for Mesa 4.x, target the [`main`](https://github.com/projectmesa/mesa-examples/tree/main) branch (default)
- To update/add an example for Mesa 3.x, target the [`mesa-3.x`](https://github.com/projectmesa/mesa-examples/tree/mesa-3.x) branch

Checkout our [HISTORY.md](https://github.com/mesa/mesa/blob/main/HISTORY.md) for the latest release notes.

This issue documents the mesa-3.x branch policy rather than naming a code or documentation change. Read the linked mesa-3.x branch and HISTORY.md first; the work is complete only when the branch targets for Mesa 3.x and 4.x examples are accurately communicated.

Written by the indexing model from the issue text.