docs: add description for preview image

Author: cotes2020

_posts/2019-08-08-text-and-typography.md

@@ -10,6 +10,7 @@ image:
   path: /commons/devices-mockup.png
   width: 800
   height: 500
+  alt: Responsive rendering of Chirpy theme on multiple devices.
 ---
 
 This post is to show Markdown syntax rendering on [**Chirpy**](https://github.com/cotes2020/jekyll-theme-chirpy/fork), you can also use it as an example of writing. Now, let's start looking at text and typography.