docs: add Stellar multisig stealth withdrawal guide

[baedboibidex-cmyk]

Summary

Adds guides/stellar-multisig-withdrawal.mdx — a complete guide for performing stealth withdrawals from multisig Stellar source accounts.

What changed

• ** New page:** guides/stellar-multisig-withdrawal.mdx covering:
  • Stellar native multisig primer (thresholds, weights, sequence numbers)
  • Building a multisig-source-account stealth withdrawal transaction
  • Coordinating $N$ signatures (XDR sharing, per-signer signing, assembly)
  • Using SDK primitives (signWithScalar, scanAnnouncements, deriveStealthKeys)
  • Recovery scenarios (lost signer, threshold change)
  • Worked 3-of-5 end-to-end example targeting futurenet
• ** docs.json** — page registered under Guides → Operations
• ** sdk/chains/stellar.mdx** — See Also cross-link added
• ** guides/privacy-best-practices.mdx** — See Also cross-link added

Acceptance criteria

• Page committed
• 3-of-5 example uses futurenet RPC (rpc-futurenet.stellar.org) and correct network passphrase
• Linked from SDK Stellar page
• Linked from Security overview (privacy-best-practices)

Design decisions

• SDK Primitives vs. Helpers: Dedicated multisig helpers are not yet in the SDK (issue notes "SDK will gain helpers"). The guide documents the pattern using existing primitives (signWithScalar, xdr.DecoratedSignature) and notes the helper API is coming.
• Coordinator Pattern: One XDR built once, shared for signing. This is called out explicitly to prevent the common mistake of each signer building their own transaction with a different sequence number.

Follow-ups

• Update this guide once dedicated SDK multisig helpers ship
• Add futurenet Friendbot-funded example keys once helpers land for easier copy-paste testing

Closes #41

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[truthixify]

Other docs PRs are landing one by one, which means most of them now conflict on docs.json (because each registers its new page there) and sometimes guides/privacy-best-practices.mdx. Quick rebase clears it:

git fetch origin
git rebase origin/develop
# in docs.json: keep all entries from both sides
# in privacy-best-practices.mdx: keep both your addition and develop's
git push --force-with-lease

Content of the PR itself is clean — this is just the registration ordering.

[truthixify]

Cascade rebase done. 530-line multisig withdrawal guide + privacy-best-practices cross-link + docs.json registration all clean. Thanks @baedboibidex-cmyk.