Add existing demos

[thomose]

Problem to solve

As ML developer I want to try demos for the mantik platform so that I can understand how to use the platform.

Details

Proposal

• [x] Add existing demo materials
• [x] Pin mantik version
• [x] Write instructions on the usage
• [x] Add a user guide

Testing

Acceptance

• [ ] Demos can be executed by an external user
• [ ] Instructions are reviewed by an external user for understandability and completeness
• [ ] Instructions are reviewed by a developer for correctness