Accessibility and headings hierarchy for the "Getting started" page.

Closes #15946 by merging it.

docs/_includes/getting-started/license.html

@@ -2,12 +2,12 @@
   <h1 id="license-faqs" class="page-header">License FAQs</h1>
   <p class="lead">Bootstrap is released under the MIT license and is copyright {{ site.time | date: "%Y" }} Twitter. Boiled down to smaller chunks, it can be described with the following conditions.</p>
 
-  <h4>It requires you to:</h4>
+  <h2>It requires you to:</h2>
   <ul>
     <li>Keep the license and copyright notice included in Bootstrap's CSS and JavaScript files when you use them in your works</li>
   </ul>
 
-  <h4>It permits you to:</h4>
+  <h2>It permits you to:</h2>
   <ul>
     <li>Freely download and use Bootstrap, in whole or in part, for personal, private, company internal, or commercial purposes</li>
     <li>Use Bootstrap in packages or distributions that you create</li>
@@ -15,7 +15,7 @@
     <li>Grant a sublicense to modify and distribute Bootstrap to third parties not included in the license</li>
   </ul>
 
-  <h4>It forbids you to:</h4>
+  <h2>It forbids you to:</h2>
   <ul>
     <li>Hold the authors and license owners liable for damages as Bootstrap is provided without warranty</li>
     <li>Hold the creators or copyright holders of Bootstrap liable</li>
@@ -24,7 +24,7 @@
     <li>Use any marks owned by Twitter in any way that might state or imply that you created the Twitter software in question</li>
   </ul>
 
-  <h4>It does not require you to:</h4>
+  <h2>It does not require you to:</h2>
   <ul>
     <li>Include the source of Bootstrap itself, or of any modifications you may have made to it, in any redistribution you may assemble that includes it</li>
     <li>Submit changes that you make to Bootstrap back to the Bootstrap project (though such feedback is encouraged)</li>