CSS: Add ::selection pseudo-element document (#23750)

This is a basic introduction to the ::selection pseudo-element CSS selector. This document is meant to be a starting point and should be added to/expounded on.

guide/english/css/selectors/pseudo/selection/index.md

@@ -0,0 +1,28 @@
+---
+title: Selection
+---
+## Selection
+
+The ```::selection``` CSS pseudo-element styles the portion of text a user clicks and drags their cursor over (the highlighted text).
+
+General Syntax:
+
+```css
+::selection
+```
+
+## Example
+
+```css
+/* "Highlighted text will be #F25757 */
+::selection {
+  color: #F25757;
+}
+```
+
+The ```::selection``` CSS pseudo-element is useful for styling user selections to match your website/app theme.
+
+#### More Information:
+* [MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/CSS/::after)
+* [W3 Schools](https://developer.mozilla.org/en-US/docs/Web/CSS/::selection)
+* [CSS Tricks - ::selection](https://css-tricks.com/almanac/selectors/s/selection/)