Change repository to a template

[eli-miriam]

We want the repository for the ehrQL tutorial to be a template repository for users to clone into their own GitHub accounts. This gives us consistency with the other tutorials (Getting Started and Becky's dummy data workshop), allows users to commit changes back to their own repositories, and reduces codespace usage for the OpenSAFELY GitHub org.

[eli-miriam]

Steve kindly changed the repo type; I'm now about to update the docs to reflect this