[doc](lance) add doc about SHOW INDEX for lance catalog

[u70b3]

Document the SHOW INDEX support for Lance Filesystem Catalog tables added in apache/doris#66637.

What Changed

• lakehouse/catalogs/lance-catalog.mdx (EN + zh-CN)
  • Feature Overview: metadata access now lists SHOW INDEX (Filesystem Catalogs only); added a System tables (table$...) row marked "Not supported" that points users to SHOW INDEX for index metadata, aligning with the Paimon/Iceberg system-table model.
  • New "Inspect Lance Indexes / 查看 Lance 索引" section: syntax and aliases (SHOW INDEXES / SHOW KEY / SHOW KEYS), the 13-column result mapping for Lance tables (only Table, Key_name, Seq_in_index, Column_name, Index_type, and Properties carry values), nested-field paths with backtick quoting, the Properties details allowlist, output examples taken from the code PR's regression test, fail-closed semantics, the SHOW privilege requirement, and the fixed SHOW INDEX is not supported for Lance REST catalogs error.
  • Current Limitations and Recommendations: added a SHOW INDEX bullet.
• sql-manual/sql-statements/table-and-view/index/SHOW-INDEX.md (EN + zh-CN)
  • Description now covers Lance logical indexes, and notes that tables in other external catalogs (Iceberg, Paimon, ...) return an empty result, with links to their system-table docs (including Paimon table_indexes).
  • Added a Lance example.

docs/ (dev) is intentionally untouched: the Lance catalog doc was removed from dev in #4086 because apache/doris master has no Lance code (the feature lives on branch-4.1).

Version Note for Reviewers

The page states "Lance Catalog is supported starting from Apache Doris 4.2", but the code shipped on the 4.1.4 line: the Lance catalog appears in 4.1.4-rc01 (2026-08-18) and SHOW INDEX appears in 4.1.4-rc02 (2026-08-25); 4.1.3 contains neither. The new content deliberately adds no version marker of its own and inherits the page-level note. Please confirm whether the page note should be corrected to 4.1.4.

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English

Docs Checklist

• Checked by AI
• Test Cases Built
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync

[zhangstar333]

@u70b3 yes, the lance catalog is release in tag 4.2