Main concepts enhancements

[JavaScriptErika]

🌳WIP - NOT READY TO MERGE 🌳

Will close issue #967 -please refer to issue for what is still being worked on. As items are completed, PR description will be updated. Thank you!

Goal- Add inline comments and enhancements to Main Concepts to further aid in comprehension for beginners

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2. Introducing JSX

Embedding Expressions in JSX

• Add comment in code snippet to assist with comprehension //Hello, Harper Perez!

JSX is an Expression Too

• Add comments to code snippet (follows along with code example from above and what the function will return).

Specifying Attributes with JSX

• Show code example of invalid JSX

JSX Represents Objects

• Change "Babel compiles" to "Babel transpiles"
• Add "It's helpful to keep this object model in mind" as we will explore rendering React elements to the DOM[...]

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