docs: add demo app

[knutties]

Problem

There is no demo application for people to to see how CAC/Superposition works which makes it hard for people to understand its usage.

Solution

This PR adds the demo project to the main repo as a crate. It has two sample apps with one building over another.

Environment variable changes

NA

Pre-deployment activity

NA

Post-deployment activity

NA

API changes

Endpoint	Method	Request body	Response Body
API	GET/POST, etc	request	response

Possible Issues in the future

NA