Add docs of Spark SQL functions for Iceberg transforms

[manuzhang]

Feature Request / Improvement

Add docs of following Spark SQL functions for Iceberg transforms

• system.iceberg_version
• system.bucket
• system.years
• system.month
• system.days
• system.hours
• system.truncate

Query engine

Spark

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time

[VerneZhong]

Hi, I’d like to work on this documentation issue.
I can contribute with some guidance from the Iceberg community.

[manuzhang]

@VerneZhong Welcome. You may refer to examples, TestSpark***Function.java, under https://github.com/apache/iceberg/tree/main/spark/v3.5/spark/src/test/java/org/apache/iceberg/spark/sql

[VerneZhong]

@manuzhang
Hi, I’m working on documenting the built-in system functions like system.iceberg_version.
Should the documentation for such functions (e.g. usage, description, example) be written in the docs/docs/(e.g. as a Markdown file), or directly as JavaDoc comments in the implementation class (like IcebergVersionFunction)?

If there’s a preferred location or existing convention, could you point me to an example I can follow?
Thanks!

[manuzhang]

@VerneZhong I mean https://github.com/apache/iceberg/blob/main/docs/docs/spark-queries.md