[docs](lance) Document index prewarm through WARM UP SELECT

[Gabriel39]

Summary

Document Lance index prewarm through WARM UP SELECT ... PROPERTIES ("read_index_only" = "true") in the English and Chinese 4.x Lance Catalog pages, with links from the existing Data Cache warmup section. Covers prerequisites, all-index and column selection, result columns, session compute-group scope, errors/cancellation, cache lifetime, and capacity guidance. Default and explicit-false data warmup behavior stays unchanged.

Companion implementation: apache/doris#68775
Tracking issue: apache/doris#68692

Merge dependency: merge after the implementation is available. This documentation does not assert a released version for index-only warmup; the existing 4.0.2 data-warmup note is explicitly scoped to data mode.

Validation

• All four changed pages compiled successfully with the Docusaurus 3.6.3 MDX compiler.
• Repository documentation governance checks reported no errors, including no broken internal links. Remaining warnings concern existing SEO/code-fence issues and the intentional version scope below; external links are report-only.
• git diff --check passed. Syntax, selection rules, permissions, timeout behavior, and result schema were checked against the companion implementation.
• A full website build was not run; validation was scoped to changed-page compilation and repository documentation checks.

Self-review

• Goal and correctness: describes the implemented index-only mode, with distinct cache and result semantics; no correctness issue found.
• Scope: four existing pages, with matching English and Chinese additions; no unrelated rewrites.
• Information architecture and versions: Lance Catalog exists only in the 4.x English/Chinese trees. The companion implementation targets branch-4.1, so this change intentionally does not add Lance material to current, 3.x, or 2.1. The 4.x Data Cache link follows that same scope.
• Links and navigation: existing URLs and anchors are preserved; the new index-prewarm anchor and local links are checked. No sidebar or redirect changes are required.
• UI/configuration/build scripts: unchanged; visual-framework checks are not applicable.
• Validation: changed-page MDX compilation and governance checks cover this documentation change; full-site build remains CI validation.
• Usability and maintainability: documents unsupported clauses, illustrative rather than fixed output values, partial cache fills after failure, and eviction/restart behavior. No additional issue found in the added content.