[D1] Add Query Insights documentation

[ivoryibu]

Summary

Adds the Query Insights documentation page for D1 at /d1/observability/query-insights/. This page covers how to access query metrics and automated index recommendations via the Cloudflare CLI (cf), Wrangler, and REST API, and walks through applying a recommended index using D1 migrations.

Key decisions reflected in this draft:

• Dashboard UI is excluded from the "Access insights" section — general query insights (latency, volume, rows read/written) are not yet exposed in the dashboard. A dedicated subsection under index recommendations notes that recommendations are visible in the dashboard under the Queries tab, with an image placeholder to be filled once a screenshot is available.
• Cloudflare CLI (cf) and Wrangler are both listed throughout as separate tools, both supporting index recommendations.
• Includes a copyable AI-agent prompt block for users who want to pipe the raw CLI JSON into an agent to generate a migration script.
• "Handle errors" API error table removed — not included in this page.

Screenshots (optional)

Documentation checklist

• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/d1/	@irvinebroque, @ivoryibu, @joshthoward, @lambrospetrou, @rita3ko, @rts-rob, @vy-ton, @cloudflare/product-owners

[cloudflare-workers-and-pages]

🚀 Deploying Preview to Cloudflare 🚀

Preview URL: https://d1-query-insights-updates.previews.developers.cloudflare.com (commit a13d403)

This URL reflects your latest Preview deployment

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Status	Deployment URL	Commit	Updated (UTC)	See this deployment's details
Build: Success ✅ Deployment: Success ✅ View logs ↗	https://68d9324e.previews.developers.cloudflare.com	a13d403	2026-09-10T19:40:15.703Z	Visit the dashboard ↗

[github-actions]

Preview URL: https://a13d4035.preview.developers.cloudflare.com
Preview Branch URL: https://d1-query-insights-updates.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/d1/observability/query-insights/	https://d1-query-insights-updates.preview.developers.cloudflare.com/d1/observability/query-insights/