Add redirect / index to tutorials folder

[bahmutov]

Currently cannot share https://docs.cypress.io/tutorials/test-a-react-todo-app link (404) - instead have to share https://docs.cypress.io/tutorials/test-a-react-todo-app/introduction.html - we need index.html to redirect to introduction doc

[jennifer-shehane]

I'll do this, I'm working on redirects right now.