docs: Improve Notes & Transactions Quick Start page

[BrianSeong99]

Context

From devrel#126 feedback by @Dominik1999.

Improvements needed

• Add core design principle: "A transaction is defined as the state transition of a single account" — explains why 2 txs are needed for an asset transfer
• Terminology: "Bob's account discovers the note" → "Bob's client discovers the note"
• Reduce repetition: Each step repeats account/faucet creation. Consider building incrementally in one demoX.rs file
• Check sync loop: Is the consumable_notes.is_empty() polling loop still needed post-0.13?
• Expected output: "Send tokens to Bob" step should also show Bob's token balance
• Key Takeaways: Says "Notes enable private asset transfers" but the code uses public notes — fix the inconsistency