Actually include the Jekyll plugin gems.

Gemfile

@@ -4,7 +4,20 @@ gem 'base64'
 gem 'bigdecimal'
 gem 'csv'
 gem 'jekyll', '~> 4.4'
-gem 'jekyll-remote-theme'
 gem 'logger'
 gem 'rexml'
 gem 'webrick', '~> 1.7'
+
+group :jekyll_plugins do
+  gem 'jekyll-coffeescript'
+  gem 'jekyll-commonmark-ghpages'
+  gem 'jekyll-gist'
+  gem 'jekyll-github-metadata'
+  gem 'jekyll-paginate'
+  gem 'jekyll-relative-links'
+  gem 'jekyll-optional-front-matter'
+  gem 'jekyll-readme-index'
+  gem 'jekyll-default-layout'
+  gem 'jekyll-titles-from-headings'
+  gem 'jekyll-remote-theme'
+end

_config.yml

@@ -8,16 +8,16 @@ url: "https://tt-rss.org"
 
 # based upon what the github-pages gem included
 plugins:
-- jekyll-coffeescript
-- jekyll-commonmark-ghpages
-- jekyll-gist
-- jekyll-github-metadata
-- jekyll-paginate
-- jekyll-relative-links
-- jekyll-optional-front-matter
-- jekyll-readme-index
-- jekyll-default-layout
-- jekyll-titles-from-headings
+# - jekyll-coffeescript
+# - jekyll-commonmark-ghpages
+# - jekyll-gist
+# - jekyll-github-metadata
+# - jekyll-paginate
+# - jekyll-relative-links
+# - jekyll-optional-front-matter
+# - jekyll-readme-index
+# - jekyll-default-layout
+# - jekyll-titles-from-headings
 - jekyll-remote-theme
 
 # Just the Docs theme settings