docs: use derive_key_argon2 in examples, derive_key no longer exists

[Sébastien Duquette (sduquette-devolutions)]

No description provided.

[Copilot]

Pull request overview

Updates the Rust README examples to reflect the current key-derivation API (derive_key_argon2), replacing the removed derive_key usage.

Changes:

• Replace the README key-derivation example to use utils::derive_key_argon2.
• Switch the example to use Argon2Parameters::default() for configuration.

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[Copilot]

Argon2Parameters::default() generates a new random salt each time (see struct docs), so derive_key_argon2 will not be reproducible across runs unless the same parameters (especially salt) are persisted/reused. The example should either show setting a stable salt (e.g., via the builder / set_salt) or explicitly mention that callers must store/serialize the parameters to derive the same key later.

Suggested change

	let parameters = Argon2Parameters::default();
	// In a real application, generate a random salt once and persist/reuse the same parameters
	// (including the salt) whenever you need to derive the same key again.
	let mut parameters = Argon2Parameters::default();
	parameters.set_salt(b"example fixed salt value");