Updated docs

[rozyczko]

• The project roadmap is updated to mark magnetism and polarisation analysis as delivered, with references to new tutorials.

• Adds detailed tutorials for magnetism (magnetism.md), polarised data (polarized_data.md), Bayesian analysis (bayesian.md), and demo datasets (datasets.md). These are linked from the main documentation index.

• Updates the data loading tutorial to explain importing polarised experiments.

Installation and Demo Data:

• Updates all installation instructions and links to reference the latest release (v1.4.0) and corrects the build platform descriptions for Linux installers.
• Documents the included demo datasets and how to use them for learning constraint features.

Technical Clarifications:

• Expands the docstring for the flatten function in structure.py to clarify the structure dictionary keys and their meanings.