Improve Next.js Rest API example

[Mayank]

• Refractor code to match community standards
  - [ ] Remove unnecessary React.FC syntax
  - [ ] Extract Layout to _app.tsx (No need to add layout on every page)
  - [ ] Use absolute path uniformly - remove import ../.., etc.
• Update UI to reflect functionality properly
  - [ ] SignUp button should be renamed to "Add Author"
  - [ ] /signup route should be renamed to /add-author
  - [ ] On create post page, email input should be changed to select (As only those emails that exists in users table can be added there)
  - [ ] Remove legacyBehaviour and update links