docs: added tutorial on how to add classes to html elements

docs/tutorials/add-default-class-to-html.md

@@ -0,0 +1,60 @@
+# Add default class for each HTML element
+
+Many people use CSS kits like Bootstrap, Semantic UI, or others that require default name classes for HTML elements:
+
+```html
+<h1 class="ui large header">1st Heading</h1>
+<h2 class="ui medium header">2nd Heading</h2>
+<ul class="ui list">
+  <li class="ui item">first item</li>
+  <li class="ui item">second item</li>
+</ul>
+```
+
+Showdown does not support this out-of-the-box. But you can create an extension for this:
+
+```js
+const showdown = require('showdown');
+
+const classMap = {
+  h1: 'ui large header',
+  h2: 'ui medium header',
+  ul: 'ui list',
+  li: 'ui item'
+}
+
+const bindings = Object.keys(classMap)
+  .map(key => ({
+    type: 'output',
+    regex: new RegExp(`<${key}(.*)>`, 'g'),
+    replace: `<${key} class="${classMap[key]}" $1>`
+  }));
+
+const conv = new showdown.Converter({
+  extensions: [...bindings]
+});
+
+const text = `
+# 1st Heading
+## 2nd Heading
+
+- first item
+- second item
+`;
+```
+
+With this extension, the output will be as follows:
+
+```html
+​​​​​<h1 class="ui large header">1st Heading</h1>​​​​​
+​​​​​<h2 class="ui medium header">2nd Heading</h2>​​​​​
+​​​​​<ul class="ui list">​​​​​
+  ​​​​​<li class="ui item">first item</li>​​​​​
+  ​​​​​<li class="ui item">second item</li>​​​​​
+​​​​​</ul>​​​​​
+```
+
+## Credits
+
+* Initial creator: [@zusamann](https://github.com/zusamann), [(original issue)](https://github.com/showdownjs/showdown/issues/376).
+* Updated by [@Kameelridder](https://github.com/Kameelridder), [(original issue)](https://github.com/showdownjs/showdown/issues/509).

docs/tutorials/index.md

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+# Tutorials
+
+* [Add default class for each HTML element](add-default-class-to-html.md)

mkdocs.yml

@@ -41,4 +41,5 @@ nav:
   - Integrations: integrations.md
   - Extensions:
     - Overview: extensions.md
-    - Create an extension: create-extension.md
+    - Create an extension: create-extension.md
+  - Tutorials: tutorials/index.md