Add example for document

[ZENOTME]

More and more people are paying attention to iceberg-rust. I’ve noticed multiple times on Slack that people are looking for examples of related features. We need to supplement our documentation with relevant examples so that people can try out the project.

• Improve write(e.g. append file) example: https://apache-iceberg.slack.com/archives/C05HTENMJG4/p1737375260439829
• datafusion example

[xxchan]

perhaps duplicate with part of #429 :p

[jonathanc-n]

i can probably write out some datafusion examples

[ZENOTME]

i can probably write out some datafusion examples

Thanks! Feel free to send PR for this.

[jonathanc-n]

Is the example here insufficient? what more do you think shold be added?