Open PreTeXt walkthrough automatically when using template

[StevenClontz]

EDIT: actually we should open the walkthrough

---

pretext-codespace/.devcontainer/devcontainer.json

Line 64 in 8b38540

"openFiles": ["source/main.ptx"]

We should instead open the README for the purpose of using this template.

[oscarlevin]

Good idea. Or maybe the getting started walkthrough?