docs: add missing built-in utility examples in API documentation

[ncthuc2004]

Summary

• add missing examples for built-in utility types: Partial, Readonly, ReturnType, and InstanceType
• keep example style aligned with existing README API sections

Context

This addresses the IssueHunt-funded docs improvement request in #51.

Testing

• docs-only change

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #51: Improve organization and add missing examples in API documentation

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[gemini-code-assist]

Code Review

This pull request adds usage examples for the Partial, Readonly, ReturnType, and InstanceType utility types in the README.md. The review feedback suggests improving consistency with existing documentation by reusing type names in examples and ensuring consistent semicolon usage within object literals.

[gemini-code-assist]

For consistency with other shallow transformation utilities in this documentation (such as Pick, Omit, Optional, and Required), it is recommended to re-use the Props type name for the result instead of introducing PartialProps. Additionally, adding a trailing semicolon to the object literal properties aligns with the style used in the Optional and Required sections.

Suggested change

	type Props = { name: string; age: number; visible: boolean };
	// Expect: { name?: string; age?: number; visible?: boolean; }
	type PartialProps = Partial<Props>;
	type Props = { name: string; age: number; visible: boolean; };
	// Expect: { name?: string; age?: number; visible?: boolean; }
	type Props = Partial<Props>;

[gemini-code-assist]

Similar to the Partial example, using the Props type name for the result and adding a trailing semicolon to the object literal properties would maintain consistency with other shallow transformation utilities like Pick, Omit, Optional, and Required elsewhere in the README.

Suggested change

	type Props = { name: string; age: number; visible: boolean };
	// Expect: { readonly name: string; readonly age: number; readonly visible: boolean; }
	type ReadonlyProps = Readonly<Props>;
	type Props = { name: string; age: number; visible: boolean; };
	// Expect: { readonly name: string; readonly age: number; readonly visible: boolean; }
	type Props = Readonly<Props>;