docs: Enhance JsonWebTokenError section in README

[subrat0796]

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Description

Describe the purpose of this PR along with any background information and the impacts of the proposed change. For the benefit of the community, please do not assume prior context.

• Added descriptions for 'secretOrPrivateKey must have a value'
• Added descriptions for 'secretOrPrivateKey is not valid key material'

This provides clarity on two common error scenarios encountered when working with JsonWebTokenError and is documented for completeness.

Testing

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Please include any manual steps for testing end-to-end or functionality not covered by unit/integration tests.

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• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not the default branch