docs: Improve Notes & Transactions Quick Start page

## Context

From [devrel#126 feedback](https://github.com/0xMiden/miden-devrel/issues/126#issuecomment-3934020694) 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