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:

content/posts/2020-07-15-the-drone-ci-debacle/index.md

@@ -2,6 +2,7 @@
 title: "The Drone CI debacle"
 date: 2020-07-15T11:04:45+02:00
 draft: false # I don't care for draft mode, git has branches for that
+description: "The misadventures of deploying my blog"
 tags:
   - hugo
   - drone

content/posts/2020-07-16-generic-flyweight-cpp/index.md

@@ -2,6 +2,7 @@
 title: "Generic Flyweight in C++"
 date: 2020-07-16T14:28:52+02:00
 draft: false # I don't care for draft mode, git has branches for that
+description: "A no-boilerplate flyweight pattern"
 tags:
   - design pattern
   - C++

content/posts/2020-07-22-polymorphic-flyweight-cpp/index.md

@@ -2,6 +2,7 @@
 title: "Polymorphic Flyweight in C++"
 date: 2020-07-22T16:16:39+0200
 draft: false # I don't care for draft mode, git has branches for that
+description: "A no-boilerplate flyweight pattern"
 tags:
   - design pattern
   - C++

content/posts/2020-12-07-git-basics/index.md

@@ -2,6 +2,7 @@
 title: "Git Basics"
 date: 2020-12-07 18:54:31+0100
 draft: false # I don't care for draft mode, git has branches for that
+description: "The next step after the basics"
 tags:
   - git 
   - cli

content/posts/2021-01-15-plaintext-accounting/index.md

@@ -2,6 +2,7 @@
 title: "Plaintext Accounting, beancount, and fava"
 date: 2021-01-15T15:54:51+01:00
 draft: false # I don't care for draft mode, git has branches for that
+description: "Or the story of my fall into systematically logging my expenses"
 tags:
   - accounting
   - cli

content/posts/2021-02-09-the-great-nix-exode/index.md

@@ -2,6 +2,7 @@
 title: "The Great Nix Exode"
 date: 2021-02-09T20:38:51+01:00
 draft: false # I don't care for draft mode, git has branches for that
+description: "My journey into declarative infrastructure"
 tags:
   - self-hosting
   - nix

content/posts/2021-10-01-magic-conversions-in-c++/index.md

@@ -2,6 +2,7 @@
 title: "Magic Conversions in C++"
 date: 2021-10-01T14:46:14+02:00
 draft: false # I don't care for draft mode, git has branches for that
+description: "How to get the compiler to infer the correct conversion"
 tags:
   - c++
   - design-pattern