DOCS: Clearly document unsupported Iceberg Features

[sungwy]

Feature Request / Improvement

Some features that users expect from an Iceberg Client (like #1084) are not yet implemented in PyIceberg, leading to much wasted time from our users.

It will be helpful to clearly document what features are not yet supported on Iceberg to minimize frustration and wasted effort from our community.

[Gowthami03B]

Since this list is always going to be extensive and bound to changes, we can add a section in the docs that points to a say unsupported-features.md in the github repo ?

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[kevinjqliu]

This is partial done by https://iceberg.apache.org/status/ which we should update after 0.9.0 is officially released