Docs: Updated Foundational Concepts page

[critesjosh]

The goal is to make the foundational concepts page more accessible and understandable without going directly into code. In the explanation, we can link out to more detailed pages, but this page should give a "protocol researcher" a good understanding of what Aztec does (and hopes to achieve).

Major concepts (keep it to the 3 top level, data privacy, confidentiality, trustlessness, but can add more detail and subsections.) This in an inexhaustive list.

• Trustless privacy
  • Data privacy (private state)
    • Note tree / nullifier tree
      • Notes
      • UTXOs
    • Private state is isolated per key
• Confidentiality (private execution)
  • PXE
    • Stores local db of every private note (that you own) + current public state
    • Txn
      • Simulates first to assure success
      • Then runs it through circuit and generates proof
      • It then creates new commitments to new data and nullifies old data
    • Then you send:
      • Proof
      • Set of new commitments
      • Set of nullifiers
    • All 3 sent to sequencer
      • Sequencer checks them (meaning, submits commitments, nullifiers and proof, to the kernel circuit)
      • Updates the trees for the whole block
    • Proofs are then sent to proving network for recursive proving (?)
      • One final rollup proof sent to Ethereum for verification
• Trustlessness (censorship resistance, network decentralization)