Fix breadcrumb: non-clickable intermediate segments - #544

Summary

Breadcrumb segments like "Features" and "Concepts" were clickable links pointing to /docs/dataset/features which doesn't exist (404). Now intermediate segments without a corresponding page render as plain text instead of links.

Uses fs.existsSync at build time to check if each breadcrumb segment resolves to a .mdx, .astro, or index.mdx/index.astro file.

Test plan

• Go to /docs/dataset/features/experiments - "Features" should be plain text, "Dataset" should be a link
• Go to /docs/evaluation/concepts/eval-types - "Concepts" should be plain text
• Go to /docs/prism/features/routing - "Features" should be plain text, "Prism" should be a link
• Go to /docs/tracing/concepts/spans - "Concepts" should be a link (tracing/concepts has index.mdx)

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