message: added 2 new slides for multi-level cat axes
diff: <meta name="author" content="https://github.com/gitbrent">
  <meta name="website" content="https://github.com/gitbrent/PptxGenJS/">
  <meta name="version" content="3.11.0">
  - <meta name="updated" content="2022-05-01">
  + <meta name="updated" content="2022-05-21">
  <link rel="icon" href="images/favicon-32x32.png" sizes="32x32" type="image/png">
  <link rel="icon" href="images/favicon-16x16.png" sizes="16x16" type="image/png">
  <link rel="icon" href="images/favicon.png">
  </div>
  </div>
  <div class="card-body">
  - <div class="row row-cols-3 g-4 text-sm mb-4">
  + <div class="row row-cols-4 g-4 text-sm mb-4">
  <div class="col">
  <h6 class="text-primary">Bar Chart</h6>
  <ul class="mb-0">
  - <li>Slide 1: vertical and horizontal bars</li>
  + <li>Slide 1: vertical and horizontal</li>
  <li>Slide 2: grid and axis options</li>
  - <li>Slide 3: stacked and percent-stacked bars</li>
  + <li>Slide 3: stacked and percent-stacked</li>
  <li>Slide 4: lots of bars</li>
  <li>Slide 5: colors, units, formats</li>
  </ul>
  <li>Slide 16 - Various Options</li>
  </ul>
  </div>
  + <div class="col">
  + <h6 class="text-primary">Multi-Level Category Axes</h6>
  + <ul class="mb-0">
  + <li>Slide 17 - Multiple Chart Types</li>
  + </ul>
  + </div>
  + <div class="col">
  + <h6 class="text-primary">Multi-Level Category Axes</h6>
  + <ul class="mb-0">
  + <li>Slide 18 - Three Level Axes</li>
  + </ul>
  + </div>
  <div class="col">
  <h6 class="text-primary">Multi-Type Chart</h6>
  <ul class="mb-0">
  - <li>Slide 17 - Various Options</li>
  + <li>Slide 19 - Various Options</li>
  </ul>
  </div>
  <div class="col">
  <h6 class="text-primary">Chart Options</h6>
  <ul class="mb-0">
  - <li>Slide 18 - Shadows and Transparent Color</li>
  + <li>Slide 20 - Shadows and Transparent Color</li>
  </ul>
  </div>
  </div>
  