docs: add impl aliases reference page

[radik878]

• Describe what impl aliases are and how they differ from plain impls
• Document the syntax impl Name<...> = path::to::Impl<...>;
• Add basic, generic, and chained impl alias examples based on existing tests
• Explain typical usage (shorter names, re-exports, partial generic specialization)
• Call out key restrictions (RHS must be an impl/alias, cycles are rejected) and link to related sections

[reviewable-StarkWare]

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[orizi]

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