docs: add build index admin function

[discord9]

What's Changed in this PR

Document admin build_index in the current English SQL ADMIN reference:

• add build_index(table_name) to the admin function list;
• add an example for building missing indexes after adding or changing index definitions;
• describe behavior, idempotency, affected-row return value, verification via SHOW INDEX and information_schema.ssts_index_meta, and resource notes.

Scope note: this PR intentionally updates only the current English docs. Localized docs and other versioned copies are deferred until after review approval.

Checklist

• Please confirm that all corresponding versions of the documents have been revised. (Deferred intentionally: this PR is current English docs only.)
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

Checks

• git diff --check
• pnpm build

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Pull request overview

Updates the SQL ADMIN reference documentation to include the BUILD_INDEX/build_index admin function, explaining when and how to use it to (re)build missing physical indexes for existing SST files.

Changes:

• Add build_index(table_name) to the documented admin function list and keywords/frontmatter metadata.
• Add a brief example invocation in the existing “Admin Functions” example block.
• Add a new “Build Index” section with usage syntax, an end-to-end example, verification queries, and resource/idempotency notes.

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