Add Ethseer to the Block Explorers section

This pull request adds Ethseer, a beacon chain blockchain scanner developed by Migalabs, to the Block Explorers section of the Ethereum documentation. Ethseer provides in-depth insights into the beacon chain.

Author: jacobourar

public/content/developers/docs/data-and-analytics/block-explorers/index.md

@@ -26,6 +26,7 @@ You should understand the basic concepts of Ethereum so you can make sense of th
 - [EthVM](https://www.ethvm.com/)
 - [OKLink](https://www.oklink.com/eth)
 - [Rantom](https://rantom.app/)
+- [Ethseer](https://ethseer.io)
 
 ## Open source tools {#open-source-tools}