Add resource: LevlPrep

[testaolivier10-del]

Resources

• Link: https://levlprep.com
  Why it helps students: Organic chemistry students work through reaction mechanisms step by step with feedback, and every topic's notes are free without an account.

Disclosure: I'm the person who builds LevlPrep. It's freemium (notes and early lessons free, an optional one time pass for unlimited practice), so I tagged it (freemium).

I ran node scripts/generate-readme.mjs after editing data/resources.json; the resources badge moves from 579 to 581 because it was one behind before this change. All four CI checks pass locally.

Checklist

• Each entry added in the correct section, in alphabetical order (case-insensitive)
• Each entry follows the format from CONTRIBUTING.md: - **[Name](https://homepage)** - Short description.
• Each entry meets the Quality Standards: real, maintained, reputable, pricing noted
• No entry duplicates an existing one in the same section

[github-actions]

Thanks for your first PR! A couple of things that'll speed up review: place your entry in its correct alphabetical position, and make sure it meets the Quality Standards (real, maintained, reputable, with pricing noted). CI will check the entry format and alphabetical order automatically, see CONTRIBUTING.md for the full checklist.