[update] add custom font size snippet, fix sample titles

- add Related sample link for custom font size in customization.md
- lowercase snippet link titles for consistency
- fix ISheetManager -> SheetManager in whats_new.md

Author: mafanya23

docs/customization.md

@@ -192,7 +192,7 @@ spreadsheet.toolbar.data.add({
 
 ![Custom Toolbar Button](assets/custom_toolbar_button.png)
 
-**Related sample**: [Spreadsheet. Toolbar Buttons](https://snippet.dhtmlx.com/qopk6lta)
+**Related sample**: [Spreadsheet. Custom toolbar button](https://snippet.dhtmlx.com/qopk6lta)
 
 In the example below a new menuItem option is added into the "clear-group" control:
 
@@ -226,7 +226,7 @@ spreadsheet.toolbar.data.update("redo", { icon: "fa fa-redo" });
 
 ![Custom Toolbar Icons](assets/custom_toolbar_icons.png)
 
-**Related sample**: [Spreadsheet. Toolbar Icons](https://snippet.dhtmlx.com/mvnx43o0)
+**Related sample**: [Spreadsheet. Custom toolbar icons](https://snippet.dhtmlx.com/mvnx43o0)
 
 ### Deleting controls
 
@@ -257,6 +257,8 @@ spreadsheet.toolbar.data.add(
 spreadsheet.parse(dataset);
 ~~~
 
+**Related sample:** [Spreadsheet. Custom font size](https://snippet.dhtmlx.com/tffbf11g)
+
 ## Menu
 
 ### Default controls
@@ -411,7 +413,7 @@ spreadsheet.contextMenu.data.add({
 
 ![Custom Context Menu Item](assets/custom_context_menuitem.png)
 
-**Related sample**: [Spreadsheet. Context Menu](https://snippet.dhtmlx.com/atl9gd4h)
+**Related sample**: [Spreadsheet. Context menu](https://snippet.dhtmlx.com/atl9gd4h)
 
 ### Updating controls
 
@@ -454,4 +456,4 @@ spreadsheet.events.on("beforeAction", function(actionName){
 spreadsheet.parse(data);
 ~~~
 
-**Related sample**: [Spreadsheet. Custom Readonly](https://snippet.dhtmlx.com/8xcursbe)
+**Related sample**: [Spreadsheet. Custom readonly](https://snippet.dhtmlx.com/8xcursbe)

docs/whats_new.md

@@ -21,7 +21,7 @@ The new release introduces significant changes in the Spreadsheet API: there is
 ### New functionality
 
 - [React Spreadsheet wrapper](/react/) is introduced. Check related examples in the [GitHub demo repository](https://github.com/DHTMLX/react-spreadsheet-examples)
-- The [`ISheetManager`](/api/overview/sheetmanager_overview/) module is introduced. It is a centralized API for managing sheets in Spreadsheet. It is accessible via the `spreadsheet.sheets` property and replaces all the [deprecated sheet-related methods](/migration/#deprecated-methods) on the root `ISpreadsheet` instance.
+- The [`SheetManager`](/api/overview/sheetmanager_overview/) module is introduced. It is a centralized API for managing sheets in Spreadsheet. It is accessible via the `spreadsheet.sheets` property and replaces all the [deprecated sheet-related methods](/migration/#deprecated-methods) on the root `ISpreadsheet` instance.
     - new methods: [`sheets.add()`](/api/sheetmanager_add_method/), [`sheets.remove()`](/api/sheetmanager_remove_method/), [`sheets.getAll()`](/api/sheetmanager_getall_method/), [`sheets.getActive()`](/api/sheetmanager_getactive_method/), [`sheets.setActive()`](/api/sheetmanager_setactive_method/), [`sheets.clear()`](/api/sheetmanager_clear_method/), [`sheets.get()`](/api/sheetmanager_get_method/)
 - The ability to specify a [custom (user-defined) formula](/functions/#custom-formulas) via the new [`addFormula()`](/api/spreadsheet_addformula_method/) method
 - The ability to display numbers in the [scientific (exponential) notation](/number_formatting/#scientific-number-format)