chore(deps): update dependency github-pages to v231 (#260)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Gemfile

@@ -3,5 +3,5 @@ source "https://rubygems.org/"
 # GitHub Pages will only load plugins from _config.yml which means that the gems
 # here are only used for local development. Update the versions when needed to
 # match GitHub Pages, see https://pages.github.com/versions for more info.
-gem "github-pages", "~> 228"
+gem "github-pages", "~> 231"
 gem "just-the-docs", "~> 0.8.0"