Add actionmailer bug report template

Introduce Action Mailer bug report templates for contributors to reproduce
issues with failing `ActionMailer::TestCase` instances.

guides/bug_report_templates/action_mailer_gem.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require "bundler/inline"
+
+gemfile(true) do
+  source "https://rubygems.org"
+
+  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
+
+  # Activate the gem you are reporting the issue against.
+  gem "rails", "~> 7.1.0"
+end
+
+require "action_mailer/railtie"
+
+class TestMailer < ActionMailer::Base
+  def hello_world
+    @message = "Hello, world"
+
+    mail from: "test@example.com", to: "user@example.com" do |format|
+      format.html { render inline: "<h1><%= @message %></h1>" }
+      format.text { render inline: "<%= @message %>" }
+    end
+  end
+end
+
+require "minitest/autorun"
+
+class BugTest < ActionMailer::TestCase
+  test "renders HTML and Text body" do
+    email = TestMailer.hello_world
+
+    email.deliver_now
+
+    assert_dom_email do
+      assert_dom "h1", text: "Hello, world"
+    end
+    assert_includes email.text_part.body, "Hello, world"
+  end
+end

guides/bug_report_templates/action_mailer_main.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require "bundler/inline"
+
+gemfile(true) do
+  source "https://rubygems.org"
+
+  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
+
+  gem "rails", github: "rails/rails", branch: "main"
+end
+
+require "action_mailer/railtie"
+
+class TestMailer < ActionMailer::Base
+  def hello_world
+    @message = "Hello, world"
+
+    mail from: "test@example.com", to: "user@example.com" do |format|
+      format.html { render inline: "<h1><%= @message %></h1>" }
+      format.text { render inline: "<%= @message %>" }
+    end
+  end
+end
+
+require "minitest/autorun"
+
+class BugTest < ActionMailer::TestCase
+  test "renders HTML and Text body" do
+    email = TestMailer.hello_world
+
+    email.deliver_now
+
+    assert_dom_email do
+      assert_dom "h1", text: "Hello, world"
+    end
+    assert_includes email.text_part.body, "Hello, world"
+  end
+end

guides/source/contributing_to_ruby_on_rails.md

@@ -44,6 +44,7 @@ Having a way to reproduce your issue will help people confirm, investigate, and
 * Template for Action Pack (controllers, routing) issues: [gem](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_controller_gem.rb) / [main](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_controller_main.rb)
 * Template for Active Job issues: [gem](https://github.com/rails/rails/blob/main/guides/bug_report_templates/active_job_gem.rb) / [main](https://github.com/rails/rails/blob/main/guides/bug_report_templates/active_job_main.rb)
 * Template for Active Storage issues: [gem](https://github.com/rails/rails/blob/main/guides/bug_report_templates/active_storage_gem.rb) / [main](https://github.com/rails/rails/blob/main/guides/bug_report_templates/active_storage_main.rb)
+* Template for Action Mailer issues: [gem](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_mailer_gem.rb) / [main](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_mailer_main.rb)
 * Template for Action Mailbox issues: [gem](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_mailbox_gem.rb) / [main](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_mailbox_main.rb)
 * Generic template for other issues: [gem](https://github.com/rails/rails/blob/main/guides/bug_report_templates/generic_gem.rb) / [main](https://github.com/rails/rails/blob/main/guides/bug_report_templates/generic_main.rb)