add link to new learning site

Author: gregsdennis

index.md

@@ -13,3 +13,7 @@ In the sidebar to the left, you'll find
 - API reference
 
 If you have question about this site or an idea for improving it, please [open an issue](https://github.com/gregsdennis/json-everything/issues/new?assignees=&labels=documentation&projects=&template=Documentation.yml) in the main repository.
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+---
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+You might also find our [interactive learning site](https://learn.json-everything.com/) engaging!