docs: document that plugin-ideal-image changes imported image types

[MasterIfeanyi]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

This change addresses issue #8684, which reported that plugin-ideal-image changes the type of imported images but this behavior was not clearly documented. This PR adds a warning block in the Usage section to make this behavior explicit for users.

Test Plan

This is a documentation-only change. I verified the changes locally by running pnpm start and navigating to the plugin ideal-image docs page to confirm the warning renders correctly.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

Closes #8684

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