docs: Add documentation for Iceberg transaction support

[garimauttam]

Summary

• add a new Transaction support section to Iceberg connector docs
• document supported multi-statement write transactions on a single table
• document current limitations and autocommit-only operations

Test plan

• docs-only change; reviewed content against Iceberg transaction behavior in code/tests

Fixes #27243

== NO RELEASE NOTE ==

Summary by Sourcery

Documentation:

• Add a Transaction support section to the Iceberg connector documentation covering multi-statement write transactions and autocommit-only operations.

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Hey - I've reviewed your changes and they look great!

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[steveburnett]

Thank you for the documentation! Just one nit of phrasing.

[hantangwangd]

Thank you so much for this documentation. I have one suggestion for addition, otherwise looks good to me.

[steveburnett]

LGTM! (docs)

Pull updated branch, new local doc build, looks good. Thanks!

[hantangwangd]

Thanks @garimauttam, lgtm!

[steveburnett]

One CI check
arrowflight-java-tests (17, :presto-base-arrow-flight)
appears stuck since at least yesterday in pending status, "Expected - waiting for status to be reported".

You may need to rebase or push the branch to re-start this stuck CI check. If you do, re-request a review and I'll approve it then.

[hantangwangd]

Hi @garimauttam, the CI stuck for docs-only PR has been fixed. Could you please rebase to the latest master branch? Then we can move forward and merge this PR.

[yhwang]

Hi @garimauttam I saw that my change and other people's changes became your commits or were authored by you, which causes the merge conflict. I guess you merged the changes from upstream into your working branch. Rebasing is the way I suggest syncing up the upstream. If you can reset your branch to your second doc commit, rebase upstream, and force push your branch, it should be able to solve the conflict.

[steveburnett]

LGTM! (docs)

Note: The documentation is fine. Please re-request my review after you follow @yhwang's advice and I will be happy to approve it then.

[hantangwangd]

Thanks @garimauttam, lgtm!

[steveburnett]

LGTM! (docs)

Pull updated branch, new local doc build, looks good. Thanks!