Website and API Docs infrastructure

Contains the source files for https://godot-rust.github.io and the CI infrastructure to generate real-time documentation from latest commits. This CI is based on GitHub Actions and offers the following workflow:

• Supports gdnative and gdext repos.
• Triggered via push to master and pull requests.
• Runs cargo doc for the respective branch.
• Creates an overview page with all active docs.
• Deploys generated docs to the website.

The website is built with Zola, a static site generator implemented in Rust. We use a custom theme inspired by Blow, together with Tailwind CSS for styling.