soroban-client: Expand docs for advanced auth and signing flows

[manoahLinks]

Document multi-signer, delegated signer, and account abstraction usage patterns.

[dumbdevss]

@dumbdevss has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi Team, would love to work on this issue. Thanks

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[drips-wave]

Congratulations, @dumbdevss! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @dumbdevss: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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