fix: add jekyll-seo-tag gem required by minimal-mistakes theme

Author: christoph2806

docs/Gemfile

@@ -6,11 +6,13 @@ gem "jekyll-feed"
 gem "jekyll-sitemap"
 gem "jekyll-gist"
 gem "jekyll-include-cache"
+gem "jekyll-seo-tag"
 
 group :jekyll_plugins do
   gem "jekyll-paginate"
   gem "jekyll-archives"
   gem "jekyll-redirect-from"
+  gem "jekyll-seo-tag"
 end
 
 # Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem