Adding my intro to probability distributions lesson to the portal

[HeidiHendrickson]

No description provided.

[github-actions]

ACT-CMS Lesson Portal — PR check

✅ All checks passed. Your lesson validates and the portal pages render correctly. It's ready to merge once a maintainer approves.

Warnings (review, not blocking)

WARNING: introduction-to-probability-distributions related_modules references unknown lesson 'thermodynamics-fundamentals'
WARNING: introduction-to-probability-distributions related_modules references unknown lesson 'statistical-mechanics-intro'
WARNING: introduction-to-probability-distributions related_modules references unknown lesson 'computational-chemistry-basics'

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See template.yaml for the full field reference. Common pitfalls: fields like platforms, authors, and scientific_objectives must be YAML lists (- item), and recommended_platform must be exactly one name that matches an entry in platforms.