feat(md-exports): Return helpful markdown 404 for agent requests

[sergical]

DESCRIBE YOUR PR

When AI agents request a non-existent .md page (e.g., /platforms/javascript/ai-monitoring.md), they currently get a 0-byte 404 response — a dead end with no guidance on where to go next.

This adds a dynamic catch-all route at app/md-exports/[...path]/route.ts that returns a proper markdown 404 with:

• YAML frontmatter (title, url)
• Sibling page suggestions from the closest ancestor in doctree.json
• Links to llms.txt, index.md, and platforms.md

Example response for /platforms/javascript/ai-monitoring.md:

---
title: "Page Not Found"
url: "https://docs.sentry.io/platforms/javascript/ai-monitoring"
---

# Page Not Found

The page `/platforms/javascript/ai-monitoring` does not exist.

## Pages in Browser JavaScript

- [Installation Methods](https://docs.sentry.io/platforms/javascript/install.md)
- [Capturing Errors](https://docs.sentry.io/platforms/javascript/usage.md)
- [Browser AI Monitoring](https://docs.sentry.io/platforms/javascript/ai-agent-monitoring-browser.md)
...

## Find what you need

- [Site index](https://docs.sentry.io/llms.txt)
- [Documentation root](https://docs.sentry.io/index.md)
- [Platforms](https://docs.sentry.io/platforms.md)

Key design decisions:

• Static .md files in public/md-exports/ still take priority — the catch-all only fires when no pre-generated file exists
• Supports both docs.sentry.io and develop.sentry.dev via isDeveloperDocs
• doctree.json included in outputFileTracingIncludes for Vercel deployment
• Graceful fallback if doctree loading fails (generic 404 with navigation links)

Context: ~7,500 markdown rewrites/month from agent traffic (99.97% via User-Agent detection). Agent 404s on .md requests were completely invisible — no metrics, no spans, no logs — because the static file miss bypasses all instrumentation.

IS YOUR CHANGE URGENT?

• Urgent deadline (GA date, etc.)
• Other deadline
• None: Not urgent, can wait up to 1 week+

SLA

• Teamwork makes the dream work, so please add a reviewer to your PRs.
• Please give the docs team up to 1 week to review your PR unless you've added an urgent due date to it.
  Thanks in advance for your help!

PRE-MERGE CHECKLIST

• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

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Project	Deployment	Actions	Updated (UTC)
develop-docs	Ready	Preview, Comment	May 24, 2026 12:55pm
sentry-docs	Ready	Preview, Comment	May 24, 2026 12:55pm