docs: add SearchD AI visibility report

[junhodo]

Summary

Add SearchD's free AI Visibility Report under Tools & Services → Analytics. The report manually runs 20 buyer questions across ChatGPT, Perplexity, Claude, Gemini, and Google AI Overviews, then returns the transcript, brands named, cited sources, and factual errors.

Disclosure: I am Bale from SearchD and am submitting our own service. The report is manually produced, not an automated analytics platform, and availability is capped to a handful per week.

Changes

• Add one neutral service entry and canonical report URL.
• Preserve the existing list format and place SearchD near the other S-named analytics services.

Verification

• https://searchd.ai/report returned HTTP 200 without redirecting.
• Read the report page and checked the entry against its published scope and free offering.
• Checked the repository and open PRs for a duplicate SearchD entry.
• git diff --check: passed; one line added to readme.md.
• Documentation-only repository; local LSP diagnostics were unavailable because the LSP daemon could not be reached.