docs: Add documentation for client sync behavior

## Context

From [devrel#126 discussion](https://github.com/0xMiden/miden-devrel/issues/126#issuecomment-4096835293) — we don't clearly explain when and how to sync with the client.

## What's needed

- When to call sync and what it does under the hood
- Whether manual sync is still required (post-0.13 changes)
- How sync relates to note discovery and consumption

## Where it should live

Builder section, likely under Core Concepts or a Guides page.