MkDocs Iceberg README

[gwionap]

Added readme for the iceberg converters.

[Thomas-Hirsch]

I think this is a good start, but it could do with a little bit more explanation at the top, some example usage and possibly some mention in the main readme as well.

Some things worth mentioning/expanding on:

• brief description of Iceberg - it's a table type in Athena, that is created and populated through SQL queries on existing Hive Athena schemas rather than overlaying a schema onto a bucket of files in S3. It took me a little while to understand that point.
• why you need IcebergMetadata as a child of Metadata. Can I convert existing Metadata into IcebergMetadata?
• some example of workflow. I would expect the AthenaIcebergTable class is more likely be used than the SQLConverter, in large part because the former is a wrapper for the latter