### _Why are the changes needed?_
Add Iceberg connector doc for Spark SQL Engine
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [ ] Add screenshots for manual tests if appropriate
- [ ] [Run test](https://kyuubi.apache.org/docs/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#3115 from deadwind4/iceberg-spark-doc.
Closes#3069
4c9adeb0 [Luning Wang] Add merge into
119be819 [Luning Wang] update mulit engine support
eb4180d6 [Luning Wang] [KYUUBI #3069][DOC] Add Iceberg connector doc for Spark SQL Engine
Authored-by: Luning Wang <wang4luning@gmail.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>