fix: docs/Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-KRAMDOWN-585939

docs/Gemfile

@@ -8,7 +8,7 @@ source "https://rubygems.org"
 #
 # This will help ensure the proper Jekyll version is running.
 # Happy Jekylling!
-gem "jekyll", "~> 3.8.7"
+gem "jekyll", "~> 4.0.0"
 
 # This is the default theme for new Jekyll sites. You may change this to anything you like.
 gem "jekyll-theme-slate", "~> 0.1.1"
@@ -19,7 +19,7 @@ gem "jekyll-theme-slate", "~> 0.1.1"
 
 # If you have any plugins, put them here!
 group :jekyll_plugins do
-  gem "jekyll-feed", "~> 0.6"
+  gem "jekyll-feed", "~> 0.13", ">= 0.13.0"
 end
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem