posts: add 'description' back to all front matters

The upstream theme has decided to revert [1] the modification that made
me decide to remove them.

On top of that, another commit has made the `description` field useful
for the `<meta name=description>` tag [2].

This reverts commit 672c4fa6be.

[1]: https://github.com/Mitrichius/hugo-theme-anubis/pull/108
[2]: https://github.com/Mitrichius/hugo-theme-anubis/pull/107

content/posts/2020-07-14-hello-world/index.md

@@ -2,6 +2,7 @@
 title: "Hello World"
 date: 2020-07-14T18:07:56+02:00
 draft: false
+description: "Test post please ignore"
 tags:
   - test
 categories: