SQL: struct support

[kbatuigas]

Description

This pull request introduces comprehensive documentation improvements for working with nested fields in Redpanda SQL, focusing on mapping nested Protobuf or Avro structures as SQL ROW columns and querying them directly. It clarifies the use of the struct_mapping_policy option, expands the reference for the ROW data type, and adds a dedicated how-to guide for querying nested fields.

New documentation and feature explanations:

• Added a new how-to page, query-nested-fields.adoc, detailing how to map topics with nested schemas as SQL tables using struct_mapping_policy = 'COMPOUND', how to query nested fields with ROW syntax, and how to handle recursive (cyclic) schemas.
• Updated the navigation (nav.adoc) to include the new "Query Topics with Nested Fields" guide.

Improvements to ROW data type documentation:

• Expanded the ROW data type reference to document field access (by position and name), wildcard projection, lexicographic comparison, NULL checks, text conversion, and usage in GROUP BY, ORDER BY, and JOIN clauses.
• Enhanced the ROW type summary to mention its support for field access, comparisons, and use in query clauses.

Clarifications to CREATE TABLE options:

• Clarified the struct_mapping_policy option in the CREATE TABLE documentation, emphasizing that COMPOUND maps nested structures to SQL ROW columns and noting that cyclic types are only supported in JSON mode.

Resolves https://github.com/redpanda-data/documentation-private/issues/
Review deadline:

Page previews

Reference > Redpanda SQL Reference > Data Types > Row
Redpanda SQL > Query Data > Query Topics with Nested Fields

Checks

• New feature
• Content gap
• Support Follow-up
• Small fix (typos, links, copyedits, etc)

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