Chore: Migrate `LiveView Todo` App from Heroku to Fly.io

Sadly, all "free" Heroku Apps will be deleted this month ... ref: https://github.com/dwyl/learn-heroku/issues/52 🤦‍♂️

# Todo

+ [x] Decide if we _need_ to have a `PostgreSQL` instance to run the demo `App` ...
or could we use [`SQLite`](https://github.com/dwyl/technology-stack/issues/92)? or even skip persistence all together and make it stateless?
+ [x] `etso` is also an `option`: https://github.com/evadne/etso
+ [ ] Migrate app to `liveview-todo.fly.dev`
+ [ ] Update deployment docs in `README.md`
+ [ ] Add GIF of working App
+ [ ] Update URL in repo settings
+ [ ] ***`DELETE`*** Heroku App! ✂️

Start with README.md and the existing deployment setup to understand how the LiveView Todo app currently runs on Heroku and whether PostgreSQL or SQLite is used. The work is done when the app is available at liveview-todo.fly.dev, deployment documentation and the app GIF are updated, the repository URL is changed, and the Heroku app is deleted.

Written by the indexing model from the issue text.