Add mdbook-beans tasks section to docs (flakes-zvh8)

Integrate mdbook-beans preprocessor into the flakes documentation, mirroring the litmus project setup. Adds kanban and all-tasks pages.

Summary of Changes

• Added mdbook-beans flake input to flake.nix
• Updated docs/book.toml with beans preprocessor config and sidebar CSS
• Created docs/src/beans/tasks.md (all tasks) and docs/src/beans/kanban.md (active tasks)
• Copied docs/src/beans-sidebar.css from litmus for bean styling
• Updated docs/src/SUMMARY.md with Kanban and All Tasks links
• Updated modules/flake/docs.nix to include mdbook-beans and .beans data in build
• Updated mise.toml docs tasks to use nix (ensuring mdbook-beans is available)