Grid multicolumn sort (#129)

* docs(grid): multi column sorting

* docs(treelist): multi column sorting

* chore(grid): remove multi column sort from TOC, it renders ugly

* chore(grid): add multi column sorting image

Author: marin-bratanov

components/grid/images/grid-multi-column-sorting.png

@@ -50,6 +50,58 @@ Click a column header to sort by its data
 
 You can sort this grid on the different columns to see the results. The `Name` column is a string, and sorting is done according to the rules for strings, while the `ID` column sorts acording to rules for integers.
 
+
+### Multi Column Sorting
+
+To allow sorting on more than one column at a time, set the `SortMode` parameter of the grid to `Telerik.Blazor.SortMode.Multiple`.
+
+>caption Enable multi column sorting
+
+````CSHTML
+@* Try sorting by Team, then by Name to see how the multiple sorts apply *@
+
+<TelerikGrid Data=@GridData Sortable="true" SortMode="@SortMode.Multiple"
+             Pageable="true" Height="400px">
+    <GridColumns>
+        <GridColumn Field=@nameof(Employee.Name) />
+        <GridColumn Field=@nameof(Employee.Team) Title="Team" />
+        <GridColumn Field=@nameof(Employee.IsOnLeave) Title="On Vacation" />
+    </GridColumns>
+</TelerikGrid>
+
+@code {
+    public List<Employee> GridData { get; set; }
+
+    protected override void OnInitialized()
+    {
+        GridData = new List<Employee>();
+        var rand = new Random();
+        for (int i = 0; i < 15; i++)
+        {
+            GridData.Add(new Employee()
+            {
+                EmployeeId = i,
+                Name = "Employee " + i.ToString(),
+                Team = "Team " + i % 3,
+                IsOnLeave = i % 2 == 0
+            });
+        }
+    }
+
+    public class Employee
+    {
+        public int EmployeeId { get; set; }
+        public string Name { get; set; }
+        public string Team { get; set; }
+        public bool IsOnLeave { get; set; }
+    }
+}
+````
+
+>caption Numbers in the column headers indicate the order by which the grid is sorted
+
+![multiple column sorting in grid](images/grid-multi-column-sorting.png)
+
 ## Sort From Code
 
 You can sort the grid from your own code through its [state]({%slug grid-state%}).

components/grid/sorting.md

@@ -22,6 +22,8 @@ You can sort the treelist on the different columns and sorting is done according
 
 Sorting keeps the expanded/collapsed state of items. For example, if filtering brings into view a child whose parent is collapsed, you will only see the collapsed parent.
 
+You can let the user sort by more than one field by setting the `SortMode` parameter to `Telerik.Blazor.SortMode.Multiple`.
+
 >caption Enable Sorting in Telerik TreeList
 
 ````CSHTML
@@ -96,6 +98,8 @@ Click a column header to sort by its data
 
 ![](images/basic-sorting.png)
 
+
+
 ## See Also
 
   * [Live Demo: TreeList Sorting](https://demos.telerik.com/blazor-ui/treelist/sorting)