docs: refine conditional exports documentation

This began as a bigger change of guidance but has been pulled back to a simple docs refactoring.

• Moves exports sugar section up
• Makes conditional exports examples using main form directly
• Explains conditional exports sugar second
• Adds some extra guidance on conditions usage
• Cuts down on some unnecessary explanations to try aid readability
• Removes incorrect statement about export main falling back to package.json main

Resolves https://github.com/nodejs/node/issues/32107.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• [] tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines