docs: add doc comment to VaultRouter struct

contracts/vault_router/src/lib.rs

@@ -1,6 +1,12 @@
 #![no_std]
 use soroban_sdk::{contract, contractimpl, Env};
 
+/// VaultRouter is the user-facing entry point for the YieldLadder protocol.
+///
+/// It routes deposit, withdrawal, and early-exit operations to the appropriate
+/// tier vault (ault_l3, ault_l6, or ault_l12) based on the lock-up
+/// period chosen by the user. All share minting and yield accounting is
+/// delegated to the selected tier vault.
 #[contract]
 pub struct VaultRouter;
 
@@ -22,4 +28,4 @@ mod tests {
         let env = Env::default();
         let _id = env.register_contract(None, VaultRouter);
     }
-}
+}