GALLERY: Added DES in Python JB URL

[TomMonks]

Proposed updates:

• Added link to Discrete-event simulation in python Jupyter book to docs/gallery.yml
• URL: https://health-data-science-or.github.io/simpy-streamlit-tutorial
• repo: https://github.com/health-data-science-OR/simpy-streamlit-tutorial

[welcome]

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Welcome to the EBP community! 🎉

[bsipocz]

Thanks!