docs: add Rust quickstart for Stellar stealth payments

[devprom6]

Summary

• Adds guides/quickstarts/rust.mdx, a first-class Rust quickstart for building backends/CLIs that talk to the Wraith Stellar contracts directly via stellar-xdr / soroban-client.
• Covers Cargo setup with testnet/futurenet/mainnet feature flags, keypair loading, stealth key derivation (reimplemented per spec), fetching + scanning announcements, and sending a stealth payment.
• Includes a callout on where the Rust story is thinner than TS and what is planned.

Acceptance

• Rust fences marked no-check so the TS snippet gate is untouched.
• Nav entry added under a new Quickstarts group in the Guides tab (docs.json).
• Cross-linked from sdk/overview.mdx.
• Renders in Mintlify (mint dev).

Closes #131

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[truthixify]

Merged. Good Rust quickstart, and splitting the cargo features for testnet vs mainnet was the right call @devprom6.