Updated Skim document (markdown)

Skim-document.md

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 **Top row buttons** actions:  
-`▕◁` **press**: go to the previous chapter; **hold**: go to the first page of the book  
-`◁☆` **press**: go to the previous bookmark; **hold**: go to the first bookmark  
-`☆` **press**: toggle bookmark; **hold**: show the list of bookmarks  
-`☆▷` **press**: go to the next bookmark; **hold**: go to the last bookmark  
-`▷▏` **press**: go to the next chapter; **hold**: go to the last page of the book  
+<kbd>▕◁</kbd> **press**: go to the previous chapter; **hold**: go to the first page of the book  
+<kbd>◁☆</kbd> **press**: go to the previous bookmark; **hold**: go to the first bookmark  
+<kbd>☆</kbd> **press**: toggle bookmark; **hold**: show the list of bookmarks  
+<kbd>☆▷</kbd> **press**: go to the next bookmark; **hold**: go to the last bookmark  
+<kbd>▷▏</kbd> **press**: go to the next chapter; **hold**: go to the last page of the book  
 
 Tap the **progress bar** to go to the desired location in the book.  
 Reader menu `Navigation` - `Settings` – `Progress bars` sets the ToC level for ticks shown in the progress bar.
  
 **Bottom row buttons** actions on press:  
-`-10` go to 10 pages backward  
-`-1` go to one page backward  
-`##` invoke `Go to page` dialog  
-`+1` go to one page forward  
-`+10` go to 10 pages forward  
+<kbd>-10</kbd> go to 10 pages backward  
+<kbd>-1</kbd> go to one page backward  
+<kbd>##</kbd> invoke `Go to page` dialog  
+<kbd>+1</kbd> go to one page forward  
+<kbd>+10</kbd> go to 10 pages forward  
  
 SkimToWidget is a [movable element](https://github.com/koreader/koreader/pull/3636).  
 Hold the widget outside of the top row buttons to toggle its transparency.