fix(html): resolve search links from docs root

[divybot]

Fixes generated HTML search-result links when searching from symbol subpages.

The search script now derives the documentation root from the loaded search.js URL via document.currentScript.src, instead of inferring depth from the ambiguous doc-current-file meta tag.

This supports denoland/deno#25069 and denoland/deno#35014.

Tests:

• INSTA_UPDATE=always cargo test --test html_test html_doc_ -- --nocapture
• cargo test --test html_test -- --nocapture
• cargo fmt --check

[deno-cla-assistant]

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• @divybot

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