Instrument tutorial hard to find on ReadTheDocs

The [Instrument tutorial](https://pygsti.readthedocs.io/en/develop/markdown/objects/Instruments.html) is buried; see below. We should probably move it to "Examples" rather than keep under "Tutorials/Objects/Operators". Or maybe we need a separate tutorial and example?

<img width="277" height="661" alt="Image" src="https://github.com/user-attachments/assets/6fdd829c-b7d5-4163-9662-ae23d393b468" />
