SQL: Expand virtual tables reference, add SHOW/DESCRIBE for new objects

[kbatuigas]

Description

This pull request significantly expands and clarifies the Redpanda SQL documentation around catalogs, Iceberg integration, and system metadata access. The updates introduce new SQL statements for listing and describing catalogs, provide detailed documentation for the information_schema and system virtual tables, and improve clarity around privileges and filtering. The most important changes are grouped below:

New SQL Statements and Catalog Management:

• Added documentation for the SHOW CATALOGS, SHOW REDPANDA CATALOGS, SHOW KAFKA CATALOGS, and SHOW ICEBERG CATALOGS statements, including syntax, filtering behavior, and privilege requirements. (modules/reference/pages/sql/sql-statements/show-catalogs.adoc, modules/ROOT/nav.adoc) [1] [2]
• Expanded ALTER REDPANDA CATALOG to cover linking and unlinking with Iceberg catalogs, including new syntax and detailed examples. (modules/reference/pages/sql/sql-statements/alter-redpanda-catalog.adoc)
• Enhanced DESCRIBE statement documentation to support DESCRIBE ICEBERG CATALOG, clarifying output and usage. (modules/reference/pages/sql/sql-statements/describe.adoc) [1] [2]

Tables and Metadata Visibility:

• Updated SHOW TABLES documentation to distinguish between Redpanda and Iceberg tables, introduce new variants (SHOW REDPANDA TABLES, SHOW ICEBERG TABLES), and explain filtering by catalog and schema. (modules/reference/pages/sql/sql-statements/show-tables.adoc) [1] [2] [3]
• Added a new page documenting the information_schema namespace, including the role_external_relation_grants view and its columns, visibility rules, and usage examples. (modules/reference/pages/sql/information-schema.adoc, modules/ROOT/nav.adoc) [1] [2]

System Virtual Tables Enhancements:

• Greatly expanded documentation for system virtual tables, including new tables for catalogs, Iceberg catalogs, Iceberg tables, Redpanda connections, and topic mappings. Each table now has a detailed schema, description, and example queries. Privilege filtering is clarified for each table. (modules/reference/pages/sql/system-virtual-tables.adoc)

These changes provide users with clearer guidance and more powerful tools for managing and querying Redpanda and Iceberg catalogs, as well as understanding metadata and permissions across the system.

Resolves https://github.com/redpanda-data/documentation-private/issues/
Review deadline: 21 May

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Checks

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

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