wiki: add Recipe-Group-By page (#1764) Maps each per-group task requested in #1764 (enum, count, frequency, slice, sort/sortcheck, reverse, sample, luau) to an existing recipe: sqlp window functions + QUALIFY, pivotp group-by mode, sample --stratified, fill --groupby, and partition + foreach --unify for generic split-apply-combine. Adds it to _Sidebar and the Cookbook index. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>