[R2] Add MotherDuck configuration example for R2 Data Catalog

[sitasheila]

Adds a MotherDuck configuration example under R2 Data Catalog → Connect to Iceberg engines, alongside the existing DuckDB, Snowflake, PyIceberg, Spark, StarRocks, and Trino examples.

MotherDuck is a managed cloud analytics platform built on DuckDB. It attaches R2 Data Catalogs as persisted MotherDuck databases (CREATE SECRET … IN MOTHERDUCK + CREATE DATABASE … (TYPE ICEBERG, …)) to read and write Iceberg tables server-side. The example calls out two R2-specific requirements: warehouse is required (omitting it returns a 403), and the default_schema namespace must already exist in the catalog.

The new page follows the format of the sibling config examples and is auto-listed via <DirectoryListing />, so no other files change.

[cloudflare-docs-bot]

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