docs: add Stellar offline transaction signing guide

- Add guides/stellar-offline-signing.mdx covering threat model,
  three-step build-offline/sign-offline/submit-online workflow,
  SDK primitives usage, QR code and USB transfer patterns,
  hardware wallet integration with honest current limitations,
  and stealth-specific air-gap guidance for spending keys
- Register page in docs.json under Guides > Operations
- Add cross-link from sdk/chains/stellar.mdx See Also section
- Add cross-link from guides/privacy-best-practices.mdx See Also section

Closes #40

Author: Bidex-cmyk

docs.json

@@ -105,7 +105,9 @@
           {
             "group": "Operations",
             "pages": [
-              "guides/stellar-mainnet-deployment"
+              "guides/stellar-mainnet-deployment",
+            "guides/stellar-multisig-withdrawal",
+            "guides/stellar-offline-signing"
             ]
           }
         ]

guides/privacy-best-practices.mdx

@@ -170,3 +170,4 @@ Identical amounts create a fingerprint. Consider varying the amount.
 | Vary payment amounts slightly | Prevents amount-based fingerprinting |
 | Use different times of day | Avoids timezone-based profiling |
 | Consolidate stealth addresses periodically | Reduces on-chain footprint |
+- [Stellar Offline Transaction Signing](/guides/stellar-offline-signing) — keep stealth spending keys fully air-gapped using the offline signing workflow