Add docs for visible columns in TreeList (#127)

* chore(treelist): add visible column docs

* chore(treelist): updates to visible columns article

* chore(treelist): add the Visible parameter to the bound article

* chore(treelist): minor fixes

* chore(treelist): fix link

* chore(treeview): updated basic example for visible param

* chore(treelist): updated toggle visibility example

* chore(treeview): shorten code snippets and new screenshots

* chore(treelist): minor adjustments

Co-authored-by: Marin Bratanov <m.bratanov@gmail.com>

Author: svdimitr

components/grid/columns/bound.md

@@ -95,7 +95,7 @@ You can use the following properties on the bound columns:
 
 * `Locked` - (defaults to `false`) - if this parameter is set to true it [locks]({%slug grid-columns-frozen%}) the column so it is always visible to the user.
 
-* `Visible` - (defaults to `true`) - if this parameter is set to `false` it [hides]({%slug grid-columns-visible%}) the column from the Grid.
+* `Visible` - (defaults to `null`) - if this parameter is set to `false` it [hides]({%slug grid-columns-visible%}) the column from the Grid. Accepts both `bool` and `bool?` types, and `null` is treated like `true`.
 
 * `Template` - this property can also be used as an inner tag and it lets you define the [column display content]({%slug components/grid/features/templates%}#column-template). It can also point to a component name.
 

components/grid/columns/visible.md

@@ -22,7 +22,7 @@ In this article:
 
 ## Basics
 
-To hide a Grid column set the `Visible` parameter to `false`. To hide a column based on a certain condition you can pass, for example, a ternary operator or a method that returns `bool` - the app can provide an expression according to its logic (like screen size).
+To hide a Grid column set its `Visible` parameter to `false`. To hide a column based on a certain condition you can pass, for example, a ternary operator or a method that returns `bool` - the app can provide an expression according to its logic (like screen size).
 
 >caption Hide a column from the Grid. Basic example.
 

components/treelist/columns/bound.md

@@ -138,6 +138,8 @@ You can use the following properties on the bound columns:
 
 * `Width` -  (defaults to `null`) - the width of the column. See the [Dimensions]({%slug common-features/dimensions%}) article. Keep in mind that these are columns in a `<table>`, so it is often a good practice to leave one column without explicit dimensions so it can accommodate the remaining width and changes to the container size.
 
+* `Visible` - (defaults to `null`) - if this parameter is set to `false` it [hides]({%slug treelist-columns-visible%}) the column from the TreeList. Accepts both `bool` and `bool?` types, and `null` is treated like `true`.
+
 * `Template` - this property can also be used as an inner tag and it defines the [custom cell content]({%slug treelist-templates-column%}) that renders instead of the default field value the treelist puts in it.
 
 * `HeaderTemplate` - this property can also be used as an inner tag and defines the custom contents of the [header cell]({%slug treelist-templates-column-header%}) that you can render instead of the Title.