update index

Author: brozorec

content/stellar-contracts/index.mdx

@@ -28,6 +28,10 @@ for access control and contract management.
 *   **[Upgradeable](/stellar-contracts/utils/upgradeable)**: Manage contract upgrades and data migrations seamlessly.
 *   **[Cryptography](/stellar-contracts/utils/crypto/crypto)**: A set of cryptographic primitives and utilities for Soroban contracts.
 
+## Fee Abstraction
+
+*   **[Fee Abstraction](/stellar-contracts/fee-abstraction)**: Enable users to pay transaction fees in tokens (e.g. USDC) while relayers cover XLM fees.
+
 ## Security and Audits
 
 Our contracts are built with security as a top priority. You can find our audit reports [here](https://github.com/OpenZeppelin/stellar-contracts/tree/main/audits).
@@ -56,6 +60,7 @@ Similarly, utilities and other modules have their own error codes:
   * Ownable: `21XX`
   * Role Transfer (internal common module for 2-step role transfer): `22XX`
 * Accounts: `3XXX`
+* Fee Abstraction: `5XXX`
 
 ## Important Notes
 As a deliberate design choice, this library manages the TTL for temporary and persistent storage items.