Add Apache Iceberg support and Catalog support to Beam SQL documentation

[talatuyarer]

This PR adds documentation for Apache Iceberg support in Beam SQL, including both new catalog management.

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Summary of Changes

Hello @talatuyarer, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the Beam SQL documentation by introducing comprehensive guides for integrating with Apache Iceberg. It covers the new CREATE CATALOG statement for managing external data sources via Iceberg catalogs and extends the CREATE EXTERNAL TABLE documentation to include Iceberg and DataGen types, providing users with detailed instructions and examples for leveraging Iceberg's capabilities within Beam SQL.

Highlights

• New CREATE CATALOG Documentation: A new documentation page has been added for the CREATE CATALOG statement in Beam SQL, specifically detailing its use for Apache Iceberg catalogs. This includes syntax, properties, examples, and best practices for catalog management.
• Enhanced CREATE EXTERNAL TABLE Documentation: The CREATE EXTERNAL TABLE documentation has been updated to include support for iceberg and datagen table types. A comprehensive section on Apache Iceberg tables covers syntax, partitioning, read/write modes, schema mapping, and various examples.
• Apache Iceberg Integration: The changes provide detailed guidance on how to leverage Apache Iceberg's features, such as ACID transactions, schema evolution, and time travel, directly within Beam SQL through both catalog and external table definitions.

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[talatuyarer]

@ahmedabu98 please review this

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[damccorm]

R: @ahmedabu98

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[damccorm]

@ahmedabu98 could you please take a look?