More restructuring

[verytactical]

• Move DEX to Ecosystem
  • not going to describe something so complex in documentation
• Add Staking to Ecosystem
  • there's a lot of articles on staking to do here
• Move system contracts to Foundations
  • these barely have anything to do with rest of Standard contracts section
  • only a few users would have to know about this
• Drop AI article for now
  • official stance is that AI is not suitable for producing secure applications yet
  • can add this article after discussion under the corresponding issue