docs: Add comprehensive Data Write Operations and Transaction API doc…

[pramila-bishnoi]

Closes #1008

Rationale for this change

This PR addresses the gap in the PyIceberg API documentation (mkdocs/docs/api.md) by adding comprehensive guides and code examples for all available data write operations and the Transaction API.

Are these changes tested?

N/A. These are documentation-only changes. No functional code changes were introduced.

Are there any user-facing changes?

Yes. This change significantly improves the documentation by providing:

• Completeness: Full documentation blocks for overwrite(), delete(), dynamic_partition_overwrite(), upsert(), and the Transaction API.

[pramila-bishnoi]

Hiii
I just opened this PR to address the missing documentation for Data Write Operations (related to #1008).
Please let me know if there are any immediate changes or formatting suggestions you would like me to address. Happy to iterate on the documentation!

Thanks for your review.