Move post-processing css to its own file

docusaurus/docs/post-processing-css.md

@@ -0,0 +1,43 @@
+---
+id: post-processing-css
+title: Post-Processing CSS
+---
+
+This project setup minifies your CSS and adds vendor prefixes to it automatically through [Autoprefixer](https://github.com/postcss/autoprefixer) so you don’t need to worry about it.
+
+Support for new CSS features like the [`all` property](https://developer.mozilla.org/en-US/docs/Web/CSS/all), [`break` properties](https://www.w3.org/TR/css-break-3/#breaking-controls), [custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables), and [media query ranges](https://www.w3.org/TR/mediaqueries-4/#range-context) are automatically polyfilled to add support for older browsers.
+
+You can customize your target support browsers by adjusting the `browserslist` key in `package.json` accoring to the [Browserslist specification](https://github.com/browserslist/browserslist#readme).
+
+For example, this:
+
+```css
+.App {
+  display: flex;
+  flex-direction: row;
+  align-items: center;
+}
+```
+
+becomes this:
+
+```css
+.App {
+  display: -webkit-box;
+  display: -ms-flexbox;
+  display: flex;
+  -webkit-box-orient: horizontal;
+  -webkit-box-direction: normal;
+  -ms-flex-direction: row;
+  flex-direction: row;
+  -webkit-box-align: center;
+  -ms-flex-align: center;
+  align-items: center;
+}
+```
+
+If you need to disable autoprefixing for some reason, [follow this section](https://github.com/postcss/autoprefixer#disabling).
+
+[CSS Grid Layout](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout) prefixing is disabled by default, but it will **not** strip manual prefixing.
+If you'd like to opt-in to CSS Grid prefixing, [first familiarize yourself about its limitations](https://github.com/postcss/autoprefixer#does-autoprefixer-polyfill-grid-layout-for-ie).<br>
+To enable CSS Grid prefixing, add `/* autoprefixer grid: on */` to the top of your CSS file.