Intro to React - the what and why

[hfaerber]

Changes made:

• Noted that students might see class-based components in legacy code but we'll talk about that later
• Updated framework vs library content to match React docs
• Removed section on functional vs class components

Will close #634

[sertmer]

This is a great update, Heather! Things that usually trip our students up are explained thoroughly here and in a variety of ways! Thanks for making these updates!

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