Use Babel and ES6 in docs JS files

Created by: NotWoods

Fixes #30818 (closed)

Updates the assets/js path inside of site so that the code is passed through Babel, using the root Babel RC file.

Additionally adds a ESLint config in the assets folder to enable a higher ecma version, but only for doc files.

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I didn't pass extra_js scripts through Babel, since the equivalent extra_css scripts in the Examples folder don't get processed. I can add this if desired.

Preview: https://deploy-preview-31607--twbs-bootstrap.netlify.app/

TODO:

• Revisit if we need babel after all
• Array.from instead of Array.prototype.slice.call
• Look into inheriting from root .eslintrc.json