Document the date_part() function

[CharlieSYH]

Related code PR

pr: risingwavelabs/risingwave#8830 (check the Documentation section in the description)

issue: risingwavelabs/risingwave#8817 (provides detailed info)

Which part(s) of the docs might be affected or should be updated? And how?

In https://www.risingwave.dev/docs/current/sql-function-datetime/, add the date_part() function.

Reference

Postgres: https://www.postgresql.org/docs/current/functions-datetime.html

Tutorial: https://www.postgresqltutorial.com/postgresql-date-functions/postgresql-date_part/