docs: Add short section about legacy bridge vs 2.0 multi-bridge suppo…

…rt (#1149)

* Add short section about legacy bridge vs 2.0 multi-bridge support

* Edits.

* Tightening up text

components/Starknet/modules/tools/pages/starkgate_architecture.adoc

@@ -38,6 +38,51 @@ The _StarkGate Registry_::
 * contains the addresses to all supported bridges
 * enables a bridge developer to stop servicing an existing bridge
 
+[#legacy_bridge]
+== Legacy bridges
+
+All token bridges that existed prior to StarkGate 2.0 (Mainnet: January 2024) besides supporting all StarkGate 2.0 functionality, are backward compatible. Each token was bridged with a unique, custom pair of L1 and L2 bridge contracts. The L1 bridge is an instance of link:https://github.com/starknet-io/starkgate-contracts/blob/cairo-1/src/solidity/LegacyBridge.sol[`LegacyBridge.sol`], and the L2 bridge is an instance of link:https://github.com/starknet-io/starkgate-contracts/blob/cairo-1/src/cairo/legacy_bridge_eic.cairo[`legacy_bridge_eic.cairo`].
+
+[#example]
+=== Example: USDC contracts
+
+* L1 ERC-20 contract address: `0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48`
+* L2 ERC-20 contract address: `0x053c91253bc9682c04929ca02ed00b3e423f6710d2ee7e0d5ebb06f3ecf368a8`
+* L1 bridge contract address: `0xF6080D9fbEEbcd44D89aFfBFd42F098cbFf92816`
+* L2 bridge contract address: `0x05cd48fccbfd8aa2773fe22c217e808319ffcc1c5a6a463f7d8fa2da48218196`
+
+Consider the `Deposit` functions for the L1 bridge contract for USDC on Etherscan: link:https://etherscan.io/address/0xf6080d9fbeebcd44d89affbfd42f098cbff92816#writeProxyContract[`0xf6080d9fbeebcd44d89affbfd42f098cbff92816#writeProxyContract`]
+
+This contract has the following two `deposit` functions: `deposit (0x0efe6a8b)` and `deposit (0xe2bbb158)`, which shows that the bridge includes support for the legacy functionality as well as the modern functionality:
+
+[horizontal,labelwidth="25"]
+`deposit (0x0efe6a8b)`:: The StarkGate 2.0 contract, which includes support for all tokens within a single contract, requiring that you enter the address of the token in the deposit function.
+`deposit (0xe2bbb158)`:: The legacy contract, which is labeled *Support Legacy ABI*. This function does not include the `token (address)` parameter, because the contract that contains this function only supports USDC. Therefore, the address of the token is superfluous.
+
+[#legacy_bridges]
+=== List of legacy bridges
+
+The following tokens have legacy contracts as well as StarkGate 2.0 contracts:
+
+* Starknet Token (STRK)
+* Starknet Voting Token (vSTRK)
+* Wrapped BTC (WBTC)
+* USD Coin (USDC)
+* Tether USD (USDT)
+* Ether (ETH)
+* Dai Stablecoin (DAI)
+* Dai Stablecoin (DAI) (Dai v0)
+* Wrapped liquid staked Ether 2.0 (wstETH)
+* Rocket Pool ETH (rETH)
+* R Stablecoin (R)
+* Frax (FRAX)
+* Frax Share (FXS)
+* Staked Frax Ether (sfrxETH)
+* LUSD Stablecoin (LUSD)
+* Uniswap (UNI)
+
+For complete details, see the link:https://github.com/starknet-io/starknet-addresses/tree/master/bridged_tokens[`bridged_tokens`] directory on GitHub.
+
 [#stark_gate_withdrawal_limit]
 == Withdrawal limit