Feature: Add BinderHub support to Launch Notebook

[mmcky]

Summary

Add BinderHub as a launch option in the Notebook Launcher (alongside Google Colab and Private JupyterHub).

Background

The previous QuantEcon theme (quantecon-book-theme) supported BinderHub as a launch target via the launch_buttons.binderhub_url configuration. The new MyST theme currently supports:

• Google Colab
• Private JupyterHub

BinderHub was intentionally deferred from the initial release to reduce scope.

Requirements

• Add a "BinderHub" option to the RadioGroup in LaunchButton.tsx
• Support configurable BinderHub URL (default: https://mybinder.org)
• Build the launch URL in the format: {binderhub_url}/v2/gh/{org}/{repo}/{branch}?urlpath=tree/{repo_subpath}{pagename}.ipynb
• Parse the repository URL from the project manifest (project.github)

Reference

Old theme implementation: https://github.com/QuantEcon/quantecon-book-theme/blob/main/src/quantecon_book_theme/launch.py

Priority

Nice-to-have for initial release. Can be added in a follow-up.