Clear up frontend examples

[Juho Teperi]

• Merge frontend-controller example to main example (or make it the main example)
• Merge frontend-prompt example to main example (but probably using react-use hooks, #374)
• Merge or remove frontend-links, not sure if there is recommended patterns on that example
• Merge frontend-auth parts to main example
• Re-frame example??? Might be OK to keep it separate.