Update dependency management to pixi

[OleBialas]

This PR migrates NeuroAI course dependency management from requirements.txt/environment.yml to a repo-local pixi environment.

Key changes:

• Added pixi.toml and pixi.lock as the source of truth for dependencies.
• Removed requirements.txt and environment.yml.
• Updated CI to install and use the locked pixi ci environment.
• Added nmaci as a locked pixi dependency and removed runtime pip install/branch override logic.
• Split default user dependencies from CI-only dependencies via a pixi ci environment.
• Kept required EGL backend libraries installed through apt for GitHub Actions rendering compatibility.
• Replaced the apt/dpkg Humor Sans font install with a direct .ttf download and font cache refresh.
• Added Colab/Kaggle guards around notebook install cells in source/project notebooks.
• Moved Macrocircuits.ipynb runtime installs into the guarded install cell and added dm-control/dm-acme[envs] to pixi.
• Updated README/book docs and workflow cache keys to use pixi.

CI result:

• Pixi setup succeeds.
• Notebook processing and finalize jobs run successfully.
• Remaining notebook failures are known runtime/content issues, not dependency setup regressions.

[iamzoltan]

Lets separate the PRs

[iamzoltan]

we should probably update this readme in general to remove the need to clone the repo and just let pixi install the packages