Convert "static page" controller specs to system specs (#31599)

spec/system/about_spec.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'About page' do
+  it 'visits the about page and renders the web app' do
+    visit about_path
+
+    expect(page)
+      .to have_css('noscript', text: /Mastodon/)
+  end
+end