Add GUI runner and packaging docs

docs/docs/content/01-getting-started/05-briefcase.md

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+# Packaging the GUI with Briefcase
+
+This project uses [BeeWare's Briefcase](https://beeware.org) to generate
+platform‑native installers. Once your development environment is set up,
+package the GUI by running the following commands from the repository root:
+
+```bash
+# Create the application scaffold for your platform
+briefcase create
+
+# Compile dependencies and produce a distributable bundle
+briefcase build
+
+# Run the packaged application
+briefcase run
+```
+
+`briefcase create` only needs to be executed once per platform. After the
+initial creation step you can repeatedly run `briefcase build` followed by
+`briefcase run` to test your packaged application on Windows, macOS or Linux.