[Docs] Use native structuredClone instead of clone-deep

[davidsandoz]

Description

As a developer using Storyblok SDK, I want to limit the amount of external dependencies so that the bundle size of my app is as small as possible, and to reduce the possible points of failure.

In the Rendering Rich Text section of the README, there's an example featuring the usage of the clone-deep library.

However, there's a native function structuredClone that is widely available to all browsers.

Suggested solution or improvement

If there's no specific reason for using the clone-deep library instead of the native structuredClone function, replace the usage of clone-deep with structuredClone.

If there's a good reason, then provide an explanation in the README.

Additional context

No response

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