docs: clarify scriptLoading behavior for existing script tags

[chenjiahan]

Summary

• Explain that scriptLoading option only affects Rsbuild-generated script tags, not existing ones in HTML template or added via html.tags/modifyHtmlTags.
• Add test cases to verify this behavior.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR clarifies the behavior of the scriptLoading configuration option in Rsbuild documentation by explicitly stating it only affects automatically generated script tags, not existing ones in HTML templates or those added via configuration APIs.

• Added documentation sections explaining the scope limitations of scriptLoading
• Created test cases to verify that existing script tags remain unchanged
• Added tests for both HTML template scripts and html.tags configured scripts

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
website/docs/en/config/html/script-loading.mdx	Added English documentation clarifying scriptLoading scope limitations
website/docs/zh/config/html/script-loading.mdx	Added Chinese documentation clarifying scriptLoading scope limitations
e2e/cases/html/script-loading/src/index.html	Created HTML template with existing script tag for testing
e2e/cases/html/script-loading/index.test.ts	Added test cases verifying scriptLoading behavior with existing scripts

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