Add tutorials and examples: Security Challenges

[sandragcarrillo]

Add three security challenges:

• cairo-damn-vulnerable-defi - Cairo and Starknet challenges inspired by Capture the Ether.
• Starknet-Security-Challenges - Cairo and Starknet challenges inspired by Capture the Ether. -
• not-so-smart-cairo - Examples of common Cairo smart contract vulnerabilities by Trail of Bits.