[guides] Add an inital MeltGuide (#2544)

Co-authored-by: anjakefala <anja.kefala@gmail.com>

visidata/guides/MeltGuide.md

@@ -0,0 +1,52 @@
+---
+sheettype: Sheet
+---
+# Melt is just unpivot
+
+Melt collapses the pivoted columns back into "Variable" and "Value" columns, converting from "wide" format into "long" format. After a melt, there is one row for each value in the pivot table.
+
+1. [Optional] remove a column from the output by hiding it: [:keys]-[/]
+2. [Optional] set key columns to keep them unmelted: [:keys]![/]
+
+3a. {help.commands.melt}
+
+A regex melt partially melts the pivoted columns. Provide a regular expression to separate the key column from other columns.
+
+3b. {help.commands.melt_regex}
+
+## Examples
+
+Start with a pivot table, created by pivoting a table with columns `R`,`B`, and key column `date`. There are two aggregators `sum` and `mean`:
+
+   date        sum_R  mean_R  sum_B  mean_B
+   ----------  -----  ------  -----  ------
+   2024-09-01  30     30      0
+   2024-09-02  0              28     28
+   2024-09-03  100    100     132    66
+
+Press `Shift+M` for a standard melt:
+
+   date        Variable  Value
+   ----------  --------  -----
+   2024-09-01  sum_R     30
+   2024-09-01  mean_R    30
+   2024-09-01  sum_B     0
+   2024-09-02  sum_R     0
+   2024-09-02  sum_B     28
+   2024-09-02  mean_B    28
+   2024-09-03  sum_R     100
+   2024-09-03  mean_R    100
+   2024-09-03  sum_B     132
+   2024-09-03  mean_B    66
+
+Or press `gShift+M` and then type a `regex` of `(\w+)_(\w)` to separate the aggregator from the column. Only the aggregator will be fully melted. The columns `R` and `B` will remain:
+
+   date        Variable  B    R
+   ----------  --------  ---  ---
+   2024-09-01  sum       0    30
+   2024-09-01  mean           30
+   2024-09-02  sum       28   0
+   2024-09-02  mean      28
+   2024-09-03  sum       132  100
+   2024-09-03  mean      66   100
+