docs: Add documentation for note tags and NoteAttachments

## Context

From [devrel#126 discussion](https://github.com/0xMiden/miden-devrel/issues/126#issuecomment-4096835293) — we don't currently explain how to use note tags or NoteAttachments anywhere in the docs.

## What's needed

- What note tags are and when to use them
- How to set and filter by note tags
- NoteAttachments: what they are, how to use them
- Practical examples

## Where it should live

Builder section, likely under the Notes concept page or the "Miden in Rust" book.