Add material database and Ashby maps

[chemiskyy]

This pull request introduces comprehensive documentation and example scripts for the new simcoon.ashby subpackage, which provides tools for material property exploration, Ashby diagram plotting, database integration, and bridging material data to simulation tensors. It also adds optional dependencies for plotting and database features, and integrates the Ashby module into the main documentation navigation.

Documentation additions:

• Added a suite of documentation files (docs/ashby/index.rst, material.rst, data.rst, plotting.rst, bridge.rst, database.rst) covering the Ashby module's data model, dataset loaders, plotting functions, bridge to simulation tensors, and Materials Project database integration. [1] [2] [3] [4] [5] [6]
• Integrated the Ashby module documentation into the main docs navigation by adding ashby/index to the docs/index.rst table of contents.

Example scripts and gallery:

• Added an example script (examples/ashby/ashby_diagram.py) demonstrating the full Ashby workflow: loading datasets, plotting diagrams, converting materials for simulation, and fetching from online databases.
• Created a README for the Ashby example gallery outlining the types of examples included.

Dependency and setup changes:

• Added an ashby optional dependency group in pyproject.toml for plotting and database features (mp-api, scipy, matplotlib).

Legacy code removal:

• Removed the legacy plotting helper functions from python-setup/simcoon/ashby.py, as they are now documented and maintained elsewhere.