docs(geneva): add multi-output UDF docs

[BubbleCal]

Feature

What is the new feature?

Adds public Geneva documentation for multi-output UDFs on the UDF guide page (/geneva/udfs/udfs). The docs now emphasize the main user benefit: one expensive input read/decode can produce multiple output columns, such as image height, width, and embedding.

Why do we need this feature?

Users may otherwise create separate UDFs for each image-derived feature, causing the same image to be read or decoded multiple times. Multi-output UDFs let users compute related features together and materialize them as sibling columns.

How does it work?

• Adds a “Registering Multi-Output UDFs” section under “Registering Features with UDFs”.
• Shows an image example that decodes the image once and returns height, width, and embedding.
• Documents typing.NamedTuple output shapes and geneva.Columns[YourNamedTuple] return annotations.
• Documents direct multi-column registration with tbl.add_columns(image_features).
• Documents prefixed output names with tbl.add_columns(geneva.UnpackedUDF(image_features, prefix="image_")).
• Clarifies that sibling output columns should be backfilled, dropped, or altered as one logical group.

Validation

• git diff --check
• npx -y mintlify@latest broken-links

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Project	Status	Preview	Updated (UTC)
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[justinrmiller]

lgtm