docs: add Stellar payment links guide

- Add guides/stellar-payment-links.mdx covering URL schema, optional
  HMAC-SHA256 tamper-protection signature, deep-linking into native
  apps, QR code generation, expiry handling, StellarPaymentLink
  reference implementation walkthrough, and webhook receiver patterns
- 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 #37

Author: Bidex-cmyk

docs.json

@@ -105,7 +105,8 @@
           {
             "group": "Operations",
             "pages": [
-              "guides/stellar-mainnet-deployment"
+              "guides/stellar-mainnet-deployment",
+            "guides/stellar-payment-links",
             ]
           }
         ]

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 Payment Links](/guides/stellar-payment-links) — avoid linking payment references to identities when generating invoices