docs(material): explain wrapping form controls

[Gaalbu]

Add a focused example showing how to wrap an existing Material control while preserving the parent FormControl state. Clarify when to use this pattern versus implementing MatFormFieldControl.

Closes #23850

Validation:

• git diff --check
• isolated documentation change; Angular dependencies are not installed in this checkout.

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[Gaalbu]

Closing this contribution because the repository requires the Google CLA and the CLA check cannot pass for this account. The change will not be pursued here.