fix: docs/Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-7164639
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-8732769
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-8732779
- https://snyk.io/vuln/SNYK-RUBY-REXML-7577227
- https://snyk.io/vuln/SNYK-RUBY-REXML-7814166
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-9510795
- https://snyk.io/vuln/SNYK-RUBY-REXML-7577228
- https://snyk.io/vuln/SNYK-RUBY-REXML-8309365
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-6228056
- https://snyk.io/vuln/SNYK-RUBY-REXML-6861566
- https://snyk.io/vuln/SNYK-RUBY-REXML-7462086
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-9510789
- https://snyk.io/vuln/SNYK-RUBY-ACTIVESUPPORT-5851458
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-9789079
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-8453714

Author: snyk-bot

docs/Gemfile

@@ -4,9 +4,9 @@ source "https://rubygems.org"
 
 # gem "rails"
 gem 'wdm', '>= 0.1.0' if Gem.win_platform?
-gem "github-pages", group: :jekyll_plugins
+gem "github-pages", ">= 229", group: :jekyll_plugins
 gem "webrick", "~> 1.8"
-gem "jekyll-avatar"
+gem "jekyll-avatar", ">= 0.8.0"
 gem "jekyll-sitemap"
 gem "jekyll-optional-front-matter", "~> 0.3.2"
 gem "jekyll-gist"