Document USDT0 for Developers under Open Source Solutions

[ed-marquez]

This was announced yesterday.

• Hedera integrates USDT0 for crosschain stablecoin liquidity:
• https://hedera.com/blog/hedera-integrates-usdt0-for-crosschain-stablecoin-liquidity/

There should be developer documentation on how to use this:

• https://docs.hedera.com/hedera/open-source-solutions