GH-44246: [JS] encourage more efficient APIs in the examples

[domoritz]

• GitHub Issue: [JS] Table.toArray() allocates a large array. arrow-js#48
• GitHub Issue: ARROW-87: [C++] Add all four possible ways to encode Decimals in Parquet to schema conversion #48

[swallez]

New section looks nice, but what about moving it at the top so that people see it first, and then have the row iterator and array examples be clearly marked as "if you need to access to rows [as objects]" to further outline that this is provided as a convenience?

[domoritz]

I changed the array example to use [...table] but only for the console.table example. Right below we now show how to access the column vectors. I think that's pretty immediate.

[github-actions]

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