Document Iceberg time travel

[colebow]

We should document how to use Iceberg time travel, as it was implemented in #10258
cc @findepi

[findepi]

FWIW The syntax and engine-side was added in #8773

[mosabua]

@colebow you can work with @findinpath and @claudiusli on that..

[tnatssb]

Any progress on this?

[findinpath]

@colebow CPTAL at #10515 ?