add blockscout as an open source block explorer

Author: austintgriffith

public/content/developers/docs/apis/backend/index.md

@@ -55,6 +55,9 @@ These libraries abstract away much of the complexity of interacting directly wit
 **Etherscan - Block Explorer and Transaction APIs**
 - [Documentation](https://docs.etherscan.io/)
 
+**Blockscout - Open Source Block Explorer**
+- [Documentation](https://docs.blockscout.com/)
+
 **GetBlock-** **_Blockchain-as-a-service for Web3 development_**
 
 - [GetBlock.io](https://getblock.io/)