Add section on inappropriate intimacy between classes

Classes should interact through public APIs, not by reaching into each other's private attributes.

**Example from MIKE IO plotting refactor:**

Before (inappropriate intimacy):
```python
# Plotter reaches into geometry internals for subsetting
values = values[self.da.geometry.top_elements]
geometry = self.da.geometry.geometry2d

# Plotter uses private attribute to check for time axis
if self.da._has_time_axis:
return self.da.values[0]
```

After (using public API):
```python
# Use sel/isel for subsetting
da = da.sel(layers="top")
geometry = da.geometry

# Use public dims property
da = self.da.isel(time=0) if "time" in self.da.dims else self.da
```

**Key principles:**
- Never access private attributes (prefixed with `_`) of another class
- Use public methods like `sel`, `isel`, and public properties like `dims`
- If you need to access private state, that's a signal the class is missing a public API
- Plotters should plot, not subset — keep data manipulation in the data layer

Start by locating the course documentation section covering class design or plotting, since the issue does not name a file or entry point. Add the provided explanation and MIKE IO examples there, preserving the guidance on public APIs and data manipulation; done means the new section is published in the appropriate course material.

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