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

Closes #15946 by merging it.

docs/_includes/getting-started/disabling-responsiveness.html

@@ -4,7 +4,7 @@
   <p class="lead">Bootstrap automatically adapts your pages for various screen sizes.
     Here's how to disable this feature so your page works like <a href="../examples/non-responsive/">this non-responsive example</a>.</p>
 
-  <h3>Steps to disable page responsiveness</h3>
+  <h2>Steps to disable page responsiveness</h2>
   <ol>
     <li>Omit the viewport <code>&lt;meta&gt;</code> mentioned in <a href="../css/#overview-mobile">the CSS docs</a></li>
     <li>Override the <code>width</code> on the <code>.container</code> for each grid tier with a single width, for example <code>width: 970px !important;</code> Be sure that this comes after the default Bootstrap CSS. You can optionally avoid the <code>!important</code> with media queries or some selector-fu.</li>
@@ -14,7 +14,7 @@
   <p>You'll still need Respond.js for IE8 (since our media queries are still there and need to be processed).
     This disables the "mobile site" aspects of Bootstrap.</p>
 
-  <h3>Bootstrap template with responsiveness disabled</h3>
+  <h2>Bootstrap template with responsiveness disabled</h2>
   <p>We've applied these steps to an example. Read its source code to see the specific changes implemented.</p>
   <p>
     <a href="../examples/non-responsive/" class="btn btn-primary">View non-responsive example</a>