docs: clarify ConditionalRender overrides

[coyaSONG]

Summary

• show where Component.ConditionalRender() belongs in a complete quartz.ts layout override
• document the required internal-component and installed-plugin imports
• use a type-checked custom slug predicate and explain that a slot override replaces that slot's defaults

Closes #2480

Validation

• the documented override type-checks with the repository's installed Search plugin
• TypeScript check passed under Node 24.14.0
• repository Prettier check passed
• all 109 repository tests passed
• documentation build completed and emitted 313 files
• git diff --check passed

This PR was written entirely using an LLM.

[github-actions]

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