[docs] Add Variant storage documentation

[JingsongLi]

What changed

• add a new multimodal-table document for Variant storage
• compare Variant with JSON stored as a string across semantics, storage, and queries
• document plain and shredded physical layouts
• describe explicit and inferred shredding schemas, query pushdown, and limitations
• add Variant Storage to the multimodal-table overview

Why

Paimon supports Variant binary storage and Variant shredding, but the multimodal documentation did not provide a focused explanation of when to use Variant instead of a JSON string or how shredding changes physical reads.

User impact

Users now have a single guide for choosing Variant, configuring shredding, and understanding the Spark and Flink version and Parquet format requirements.

Validation

• git diff --check
• npx --yes yarn build

The Docusaurus build completed successfully. It reported the existing site-wide broken link warning for /docs/master/concepts/overview.