Update description of node_modules handling

[Macil]

• Updates the Transforms page to be consistent with Automatically transpile dependencies with babel-preset-env parcel#559 and [RFC] Support source field in package.json to enable babel on symlinked modules parcel#1101.
• Adds a link from the Transforms page to the Babel section of the JavaScript page. (I originally arrived to the Parcel documentation specifically curious about how Parcel handled Babel, and the "Transforms" page seemed to be the page most specifically about Babel, so I incorrectly assumed the rest of the documentation didn't have anything more detailed about Babel. I think this link would have helped me out.)

[vercel]

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