Improve discoverability and completeness of aggregation/statistics documentation

[koperagen]

Problem

Aggregation/statistics functionality is hard to discover and inconsistently documented.

Specifically:

• Statistics documentation is difficult to find from relevant pages
• Naming conventions (x, xOf, xFor) are not clearly explained in one place
• Some existing examples (e.g. from Analyze.kt) are not used in documentation

Expected

Improve discoverability and ensure documentation completeness for aggregation/statistics APIs.

Acceptance criteria

• Statistics documentation is linked from relevant pages (e.g. operations, KDocs)
• Naming conventions (x, xOf, xFor) are explained in one place and referenced
• Unused examples are either:
  • added to documentation, or
  • removed
• Documentation allows users to find statistics APIs without prior knowledge

Motivation

• Aggregation is a core part of data analysis workflows
• Poor discoverability leads to underused or misunderstood functionality
• Documentation must be navigable and complete before 1.0

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

[zaleslaw]

Moving this to 1.0.

This is documentation work for a core DataFrame workflow: aggregation and statistics. It does not require API changes, but improves discoverability of important public APIs, explains the x / xOf / xFor conventions, and helps users find the right examples during the 1.0 launch.

Since documentation and examples are the current priority for the public campaign, this fits the 1.0 milestone.