Create Monitoring tools section and add Block Explorers

[sandragcarrillo]

There is another Block Explorer no added in the README:

• NFTScan - Explore any NFT data on Starknet.

Also, we can create Monitoring tools section to include the following resources:

• Starknet monitor - vital statistics about Starknet.
• Starknet status - Provides timely status information about the health of Starknet sequencing.