Some information about aggregation functions is missing from docs / some is hard to find

[koperagen]

1. It was hard to find information about https://kotlin.github.io/dataframe/summarystatistics.html. I tried to search "naming conventions" to refresh memories about "xFor, xOf, x". Found only https://kotlin.github.io/dataframe/modify.html#naming-conventions. I also forgot Statistics even exists :o Maybe we might add links to documentation in kdocs? But i'm not sure if i ever seen people doing it and if it's a good / bad practice

2. There are samples are never used across documentation, like columnStats_properties or multipleColumnsStat and some other from Analyze.kt

[koperagen]

Apparently kdocs allow you to avoid copy-paste somehow. So it should be possible to embed this part of docs right there

When statistics x is applied to several columns, it can be computed in several modes:

x(): DataRow computes separate value per every suitable column

x { columns }: Value computes single value across all given columns

xFor { columns }: DataRow computes separate value per every given column

xOf { rowExpression }: Value computes single value across results of [row expression](https://kotlin.github.io/dataframe/datarow.html#row-expressions) evaluated for every row