document the crypto package

[MichaelMure]

This pull request introduces comprehensive documentation and a practical example for the crypto package, making it easier for developers to understand and use its features. The changes focus on explaining the package's goals, supported algorithms, and providing a step-by-step usage guide.

Documentation improvements:

• Added a detailed Readme.md for the crypto package, describing its purpose, the problems it solves compared to standard Go crypto packages, and listing supported cryptographic algorithms and their capabilities. The README includes a clear example workflow covering key generation, serialization, signing, key exchange, and configuration management using varsig.

Examples and usage:

• Added a new example in crypto/_example/example_test.go that demonstrates real-world usage of the package, including key generation, serialization, signing and verifying messages, key exchange, and use of signing configuration (varsig). This example closely matches the one provided in the README for consistency and clarity.

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Note

Improves developer-facing documentation for crypto without changing behavior.

• Adds crypto/Readme.md with package overview, usage example, and supported algorithms table
• Introduces crypto/example_test.go showcasing key generation, serialization, signing/verification, key exchange, and varsig usage
• Adds GoDoc comments and clarifications in crypto/public.go, crypto/hash.go, crypto/options.go, crypto/jwk/*, and _testsuite/testsuite.go

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