[Variant] add doc reference to VariantArrayBuilder

[sdf-jkl]

Which issue does this PR close?

• Closes [Variant] Improve ergonomics when using VariantBuilder #9113.

Rationale for this change

Users might go for VariantBuilder to manually build VariantArrays.

Instead they should use VariantArrayBuilder.

What changes are included in this PR?

• adds an example and a doc reference to VariantArrayBuilder for VariantBuilder

Are these changes tested?

n/a

Are there any user-facing changes?

doc change

[alamb]

Thanks @sdf-jkl -- this is a nice improvement

I left a suggestion for making it even better -- let me know what you think

[alamb]

I believe this is marked as ignore as the VariantArrayBuilder lives in a different crate

It seems like it would be better to not ignore this doc so that it was tested and we ensure it is a complete example.

What do you think about leaving the text above

/// `VariantBuilder` builds a single, self-contained [`Variant`] value -- useful
/// for one-off values and unit tests. To build an array (column) of variants,
/// one per input row, use [`VariantArrayBuilder`] from the
/// `parquet-variant-compute` crate rather than a `VariantBuilder` per row. 

In this crate

And then adding the example and detail about VariantBuilderExt directly as an exampel on VariantArrayBuilder - that would likely make it easier to find

/// [`VariantArrayBuilder`] implements [`VariantBuilderExt`], so you append values and 
/// nested objects or lists the same way as when building a single Variant value with [`VariantBuilder`]:
///
/// ```
/// <real example ehre
/// ```

🤔

[sdf-jkl]

964506d

That makes sense, thanks. I dropped the example because VariantArrayBuilder already has some.