[docs](variant) document ColumnVariantV2 compute behavior

[eldenmoon]

What changed

• Document the experimental ColumnVariantV2 compute path introduced by [Feature](variant) Add native ColumnVariantV2 execution doris#65561 in the current English and Chinese VARIANT pages.
• Clarify the default execution-path restrictions versus V2 canonical hashing and serialization support for grouping, distinct, set operations, nested expressions, and explode.
• Document enable_variant_v2, E/T whole-column physical states, SQL NULL versus Variant/JSON null, explicit JSON parsing functions, and the unsupported root comparison, join-key, sort, window, and MIN/MAX cases.
• Align English and Chinese ELEMENT_AT documentation with the current Variant array behavior: positive indexes are 1-based, negative indexes count from the end, and zero or out-of-range indexes return NULL.

Why

PR #65561 changes Variant execution semantics without changing table storage compatibility. The docs need to distinguish the experimental compute-only path from the existing default-path CAST guidance so users do not infer unsupported root comparisons or join keys.

Validation

• git diff --check: passed.
• corepack yarn install --frozen-lockfile: passed.
• Docusaurus build was attempted, but the repository-wide build reads all locales and historical versions and grew to approximately 16–24 GB of memory before it was stopped to protect the remote host; it emitted only pre-existing Markdown warnings before termination.