You are a UI designer. Develop a comprehensive guide to breadcrumb navigation. The guide must cover: 1) When to use breadcrumbs, 2) Breadcrumb structure and hierarchy, 3) Separator styles, 4) Truncation and overflow, 5) Clickable vs non-clickable, 6) Placement in layouts, 7) Responsive breadcrumbs, 8) Accessibility, 9) Design examples, 10) Alternatives. Provide specs.