Add KDocs for mean.kt

[Jolanrensen]

** Generated with Air/Junie **

Description

Adds full KoDEx-based KDocs for the mean operation (DataColumn.mean/meanOf, DataRow.rowMean/rowMeanOf, DataFrame.mean/meanFor/meanOf, Grouped.mean/meanFor/meanOf, Pivot.mean/meanFor/meanOf, PivotGroupBy.mean/meanFor/meanOf), following the same style already used for sum, min, and max.

Closes #1972

Changes

• core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/mean.kt: replaced the old TODO KDocs comment with a proper // region docs block (MeanDocs : CommonStatisticsDocs) and per-overload KDocs, reusing shared CommonStatisticsDocs snippets (null/NaN handling, etc.) to avoid duplicating documentation text.
• core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/documentation/DocumentationUrls.kt: added DocumentationUrls.Mean (+ TypeConversion) linking to the existing mean.html documentation page.

Verification

• ./gradlew :core:compileKotlin — success
• ./gradlew :core:ktlintMainSourceSetCheck — success
• ./gradlew :core:processKDocsMain — success, no unresolved KoDEx references
• ./gradlew :core:test --tests "*mean*" --tests "*Mean*" — success

Additionally, I checked all generated text, moved some extra KDocs to CommonStatisticsDocs, and I added some extra notes to min and max that we added to sum and could be useful there too.

[Jolanrensen]

@Allex-Nik thanks! Very good review once again :)

[Allex-Nik]

Cool! I like the docs

Only #2102 (comment) is left to process one way or another :)

[jetbrains-air]

Thanks for the thorough pass on these docs — one thing to sort out before merge!

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