Reviewed and refactored next/previous navigation button implementation

- Updated styling to include item name.
- Extracted used text to translations.
- Updated the design to better suit the surrounding blocks.
- Removed newly added model/repo methods.
- Moved core logic out of controller and instead into a "NextPreviousContentLocator"
helper with re-uses the output from the book-tree generation.
- Also added the system to chapters.

For #2511

app/Entities/Models/Page.php

@@ -138,13 +138,4 @@ class Page extends BookChild
         $refreshed->html = (new PageContent($refreshed))->render();
         return $refreshed;
     }
-    /**
-     * Get the parent chapter ID.
-     */
-    public function getParentChapter()
-    {
-        $chapterId = $this->chapter()->visible()
-        ->get('id');
-        return $chapterId;
-    }
 }