Use Pixi for CI/CD setup

[kylecarow]

Affects deploy_docs and build_and_test, not release (yet)

Changes to pyproject.toml

• Reorganize optional-dependencies into dependency-groups, this aligns better with the philosophy here
  https://pydevtools.com/handbook/explanation/what-are-optional-dependencies-and-dependency-groups/
  • no end users should be using pip install '.[dev]'/pip install fastsim[dev] so I'm not fussed about compatibility changes. You can still install these deps by doing pip install --group dev . or pip install --group dev -e . anyway.
  • I left a pymoo optional-dependency in there commented out, it seems like that would be a sensible optional dependency. The way we reference pymoo in this repo is messy anyway and should generally be cleaned up.
• project.requires-python bumped to allow Python 3.10 through 3.14
• New Pixi environments to facilitate Python version matrix

Workflows use pixi for Rust setup:

• Each currently supported Python version gets its own pixi env with specified dependency
• Reorganized deploy_docs and build_and_test workflows to use pixi for Rust installation, Python installation, FASTSim, and extra dependencies. docs workflow uses new docs pixi env that does not include any extra deps beyond what is required.
• Is Rust caching step necessary? The robots thought that line ought to go, but perhaps it's still useful?

[kylecarow]

Superseded by #265