Upgrade theme to version v0.2.0 (#5453)

Upgrades jekyll-theme-minimal to current version v0.2.0
See guidelines at: https://github.com/pages-themes/minimal#usage

Related with EbookFoundation/free-programming-books#2479

_config.yml

@@ -1,10 +1,17 @@
-theme: jekyll-theme-minimal
+# [Name of visual theme]
+#theme: jekyll-theme-minimal
+remote_theme: pages-themes/minimal@v0.2.0
+
+# [Used rubygem plugins]
 plugins:
+  - jekyll-remote-theme
   - jemoji
   - jekyll-relative-links
+
 relative_links:
   enabled: true
   collections: true
+
 include:
   - CONTRIBUTING.md
   - LICENSE.md