Redesign readme to match new style

[ewanharris]

Changes

Updates the project readme to match the new standard

• Updates readme structure
• Moves existing code samples out into a dedicated examples document

Still needs the new header image adding

Checklist

• [x] I have read the Auth0 general contribution guidelines
• [x] I have read the Auth0 Code of Conduct
• [x] All existing and new tests complete without errors