Add FAQ entries for keygen explanation and crackme safety

[xusheng6]

Summary

Adds two new FAQ entries to address common user questions:

1. What is a keygen? - Explains what keygens are and their educational value
2. Are the crackmes safe to run? - Addresses safety concerns and provides best practices

Changes

"What is a keygen?" (#what-is-keygen)

• Defines keygen as a program that generates valid serial numbers/passwords
• Explains the difference between keygens and patching
• Lists benefits: demonstrates deep understanding, educational value, reusable solution
• Notes that keygens can be written in any language and should be accompanied by a writeup

"Are the crackmes safe to run?" (#are-crackmes-safe)

• Explains that we manually review every submission to prevent malware/viruses
• Acknowledges there's always a small chance something could be missed
• Recommends best practices:
  • Use a virtual machine (VM)
  • Take snapshots before running
  • Disable network access
  • Check crackme details and comments
• Explains that antivirus detections on VirusTotal are completely normal because crackmes use techniques similar to malware (anti-debugging, obfuscation, self-modifying code, etc.)
• Links to the #report-crackme section for users who believe they found actual malware

Test plan

• Verify the FAQ page renders correctly with both new entries
• Verify anchor links (#what-is-keygen, #are-crackmes-safe) work correctly
• Verify the link to #report-crackme in the safety section works

Fixes crackmesone/crackmes.one#67
Fixes crackmesone/crackmes.one#65

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