Added non-qubit local observables

[linusschulte]

Description

Added support for one-site non-qubit observables using new LocalOperator in order to enable measurement of things like physical position.
Tests cover Observable construction from local matrices, MPS expectation evaluation on non-qubit sites, dimension-mismatch errors, and the proper expected trapped-ion trajectory.
Note: Local Observables are still called "Gates" due to qubit-only heritage. That might be a bit misleading for non-logical operators but renaming that consistently is outside of this PR's scope.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[aaronleesander]

@linusschulte Can you do a couple things

1. I don't want the user to have to import Observable. If you check previous usage, users just type Observable("name", site). Can you change it to be more user-friendly somehow? If not, let me know and we can look through it
2. There was just a release so the CHANGELOG point should be moved up under [Unreleased]. You might need to add an added header like in the previous ones.

[codecov]

Codecov Report

❌ Patch coverage is 84.12698% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mqt/yaqs/core/data_structures/mps.py	77.1%	8 Missing ⚠️
...yaqs/core/data_structures/simulation_parameters.py	85.7%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[linusschulte]

Regarding your first review point:
Are you looking for something like Observable("position", site)? The constructor needs the actual grid values, so I don't think it can be done without passing additional context. The next best thing I can think of is something like Observable.position(positions, site), but that's really just a wrapper for np.diag, and I'm not sure how much higher that ranks on the user-friendliness scale.