Infrastructure for a bikeshed-based spec.

[otherdaniel]

This lands some infrastructure for a future, bikeshed-based spec. The spec itself is a placeholder for now.

• auto-publish.yml: A workflow to build & publish the spec to GitHub pages.
• .pr-preview.json: Hook into a mechanism to auto-attach previews to GitHub pull requests.
• build.ninja: Ninja file to run bikeshed.
• permission-element.bs: A mostly empty bikeshed spec.
• .gitignore: Ignore file to ignore build artefacts in commits (and thus pull requests).