Demo Notebooks are Runnable in Binder

[yoonspark]

Summary

To help new users test out LineaPy right away, we have added another executable environment for demo notebooks: Binder. Advantages over Colab (which we still support) include:

• No need for a separate account (cf., Google account is required for Colab)
• More familiar Jupyter interface (cf., Colab presents its own flavor of IPython)

On the other hand, note that the environment startup is slower than that of Colab.

Reference

• Add Binder link for demos #690
• https://github.com/LineaLabs/demos/pull/14