Documentation page for "big rocks"

[kylebarron]

The idea is that we should document in a public place the big hurdles to making geopolars more performant and interoperable.

• Polars does not support FixedSizeList... means that we can't today use the arrow-native geometry format defined in geo-arrow-spec
• Polars does not support custom extension types... means that we can't consume data coming in from other applications that conforms to the latest draft of geo-arrow-spec that wraps geometry types like geoarrow.wkb in an ExtensionType("geoarrow.wkb", BinaryArray).
• Polars does not use the arrow BinaryArray, but rather uses List<Uint8Array>. This means the recent geozero work to make reading faster from a BinaryArray is unusable unless it's also implemented for ListArray<u8>. This also means receiving data from other applications that store it as BinaryArray requires a cast.
• Geometry traits in geo-types... means that we need at least two copies: one into the geo struct and another back into arrow memory after processing. We have a third right now that copies the arrow data into a Vec<u8>