Delete bootstrap5 branch?

Hi @deining. Now that BSv5 work is progressing over `main`, can you delete the [bootstrap5](https://github.com/google/docsy/tree/bootstrap5) branch?

> <img width="395" alt="image" src="https://user-images.githubusercontent.com/4140793/215550355-bad87818-e372-4b88-8466-534634cdbc94.png">
