`xs` (cross-section) operation missing KDocs

`xs` (cross-section) operation is still missing KDocs. I keep forgetting what it does XD

The website could also use some improvement. https://kotlin.github.io/dataframe/xs.html

What I think could help most is some rewrites into other functions, to see what `xs` is shorthand for.
For instance, that
```kt
df.xs("a", "b")
// is equivalent to
df.xs("a", "b") { df.flatten().columns().take(2) }
```
and that
```kt
df.xs(1, 2) { colA and colB }
// is equivalent to
df
.filter { colA == 1 && colB == 2 }
.remove { colA and colB }
```

Start by locating the KDoc and generated website source for the `xs` operation, then compare them with the current page at kotlin.github.io/dataframe/xs.html. Document what `xs` does and include the proposed equivalent `flatten`, `columns`, `filter`, and `remove` examples. Done means the API KDoc and website explain the operation clearly with representative rewrites.

Written by the indexing model from the issue text.