Provide requirements-freeze files as backup

[rmarquis]

While we teach how and why generating a requirements-freeze.txt file in the guide, we do not procide such files to the readers. Indeed, the objective is to teach how to do it as a best practive.

This also means that we don't have full control of transitive dependency in the guide, and breakage might occur (the latest time in #221). In practice, this means the guide might stop working between the time we last test the content, and the time the content is read.

To avoid annoying some awkward in teaching environement, we can provide pre-generated requirements-freeze.txt files, as backup in case a breakage arise.

Prepare backup requirements-freeze.txt files for each chapter, and upload them in a separate freeze-backup branch.

If needed, the content could be accessed manually or with wget:

# Download the requirement-freeze.txt backup
wget https://github.com/swiss-ai-center/a-guide-to-mlops/freeze-backup/refs/heads/requirements-freeze-chapterX.txt -O requirements-freeze.txt

Ensure this branch is correctly updated when needed.

[rmarquis]

Done, in the freeze branch.