Fix: Replace var with let and const in documentation

[OMantere]

This was mentioned in #8035

Updates var variable declarations in the documentation examples to use ES6-style const, let and import, except for the "React without ES6" page and the LinkedStateMixin example on "Two way binding helpers", as mixins are not supported in ES6.

I could also update the CodePen examples on "Thinking in React" that still use var, can I link my own fork into the docs?

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[gaearon]

Thanks!