chore: remove unused mkdocs plugin dependencies from pyproject.toml

[MAfarrag]

Context

PR #149 added the `docs` optional dependency group with mkdocs plugins.
Three plugins are listed but not configured in `mkdocs.yml`.

Problem / Current Behaviour

These packages are installed but unused:

• `mkdocs-macros-plugin`
• `mkdocs-mermaid2-plugin`
• `mkdocs-panzoom-plugin`

Affected locations

File	Symbol	Notes
`pyproject.toml:69-73`	`[project.optional-dependencies] docs`	3 unused deps

Proposed Solution

Either:

1. Remove them from `pyproject.toml` if not planned for use
2. Or add them to `mkdocs.yml` plugins list if they are needed

Effort Estimate

Size: `XS`

Definition of Done

• Unused dependencies removed or configured
• `pixi install` resolves cleanly
• `mkdocs build` succeeds

[MAfarrag]

Closed: created in error. These are follow-up items from PR #152 review, not standalone issues.