Migrate CI environment to pixi

[OleBialas]

Migrates the NeuroAI course environment and CI setup from requirements.txt / environment.yml to pixi.

• Adds locked pixi.toml / pixi.lock with default, CI, legacy book, and Jupyter Book v2 environments.
• Updates CI workflows/actions to install dependencies through pixi instead of pip requirements.
• Installs nmaci from the locked pixi environment rather than dynamically downloading/installing it.
• Removes legacy requirements.txt and environment.yml.
• Updates docs for local pixi setup and book builds.
• Cleans up Macrocircuits.ipynb runtime installs by moving required packages into the guarded hosted-notebook install cell.
• Fixes notebook-pr checkout for fork PRs and updates checkout action to v5.

[OleBialas]

Used skip ci to avoid generating the notebook artifacts for the PR