Update ergonomics across the user-guide

[avimallu]

As discussed on Discord.

After pola-rs/polars#6451, parts of the user-guide were not updated with the newer ergonomics. This does not lead to the examples not working, but leads to Polars code looking rather cluttered. In addition, it can lead to user confusion since most Polars code now doesn't have these restrictions. For example:

out = df.select(
    [
        pl.struct(["keys", "values"])
        .apply(lambda x: len(x["keys"]) + x["values"])
        .alias("solution_apply"),
        (pl.col("keys").str.lengths() + pl.col("values")).alias("solution_expr"),
    ]
)

can and should now be written as:

out = df.select(
    pl.struct(["keys", "values"])
    .apply(lambda x: len(x["keys"]) + x["values"])
    .alias("solution_apply"),
    (pl.col("keys").str.lengths() + pl.col("values")).alias("solution_expr")
)

This PR updates such cases that are scattered across the user guide's Python code files. The migrating section had some of this content within the markdown itself, so I updated the two files there as well.

[avimallu]

Conflicts resolved - simply had the column selection code from the earlier functions page.