docs: add runnable Examples sections to public functions that lack them

The documentation build executes the Examples code blocks in docstrings (the ```python exec="1" blocks), so a runnable example doubles as documentation and a lightweight smoke test. Several public functions have no Examples section.

What to do

Add an Examples section with a short, runnable snippet, matching the existing style:

```python exec="1" source="above" result="text"
import numpy as np
from toqito.states import dicke

print(dicke(...))
```

Good targets (pick one or a few per PR):

• toqito/measurement_ops/measure.py
• toqito/states/dicke.py
• toqito/state_props/has_symmetric_inner_extension.py
• toqito/state_props/integral_relative_entropy.py
• the cone builders in toqito/cones/ (e.g. geometric_mean_epi_cone.py)

Verifying

The example runs during the docs build:

uv sync --group docs
cd docs && uv run mkdocs build

Confirm the snippet runs without error and the printed output is sensible. Looking at an existing function's Examples section (e.g. toqito/channels/dephasing.py) is the easiest way to see the expected format.