Update broken links to current ones

[sandragcarrillo]

There are some links that are broken and the resource is now available in a different link:

• Katana - High performance sequencer that can be used as a local testnet. -> new link is https://book.dojoengine.org/toolchain/katana
• starknet.dart](https://starknetdart.dev/) - Dart library for mobile. -> New link: https://github.com/focustree/starknet.dart