Add documentation for nested fields

[mosabua]

for engine and sql language in general (ALTER COLUMN, CREATE TABLE .. and so on)

And then also for connectors that support this. Currently only Iceberg.

See #16959 and linked PRs and release notes.

[mosabua]

Also see discussion in #18496 - @ebyhr will help with review

[dhuynh]

CREATE TABLE for nested fields exists for JSON - is this what you are looking for?
CREATE TABLE persons ( firstName varchar, lastName varchar, address ROW( street varchar, city varchar, country varchar ), jobs ARRAY( ROW( job varchar, episode varchar ) ) ) WITH ( external_location = 's3a://data-lake/persons', format = 'JSON' )

Does this nested field syntax work across connectors and formats like Hive/Avro and Iceberg/Avro?

[mosabua]

Yes .. this is one of the things we need to document .. but we had more features around this in recent releases.