fix(guide): simplify directory structure

guide/english/css/tutorials/introduction-to-css/index.md

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+---
+title: Introduction to CSS
+---
+## Introduction to CSS
+
+### What is CSS?
+
+Cascading Style Sheets (CSS) describe how the html on a page should appear.
+
+Before CSS developers would apply styles using attributes or special tags on each node of a page. This made markup repetitive and prone to errors.
+
+CSS allows selectors to describe how each piece of matching content should look.
+
+```CSS
+body {
+    font-size: 15px;
+}
+
+a {
+    color: rebeccapurple;
+    text-decoration: underline;
+}
+```
+
+### Using CSS
+
+**External style sheets** let many pages share the same styles.
+
+```HTML
+<link href="styles.css" rel="stylesheet" type="text/css">
+```
+
+**Internal style sheets** apply CSS to all matching tags on a page.
+
+```HTML
+<style>
+    h1 {
+        font-family: sans-serif;
+        margin-bottom: 1.5em;
+    }
+</style>
+```
+
+**Inline CSS** apply styles to a single tag.
+
+```HTML
+<img src="..." style="border: 1px solid red;" />
+```
+
+#### More Information:
+* [W3Schools](https://www.w3schools.com/css/css_intro.asp)
+* [CSS-Tricks Almanac](https://css-tricks.com/almanac/)
+* [Sitepoint](https://www.sitepoint.com/html-css/?ref_source=github)