[Getting Started] Rewrite Continiouus Integration section to use bin/ci

With Rails 8.1, we have introduced `bin/ci` as the starter to CI pipelines.
Currently, the guide focused on GitHub Actions as the starter which is tied
to the specific platform.

This change rewrites the Continuous Integration section in Getting Started guide
to use `bin/ci` as the main example, making it platform-agnostic, emphasizing
local execution and introducing the recent capabilities of `bin/ci`.

Author: harsh183

guides/source/getting_started.md

@@ -2961,16 +2961,119 @@ No warnings found
 
 Learn more about [Securing Rails Applications](security.html)
 
-Continuous Integration with GitHub Actions
+Continuous Integration with `bin/ci`
 ------------------------------------------
 
-Rails apps generate a `.github` folder that includes a prewritten GitHub Actions
-configuration that runs rubocop, brakeman, and our test suite.
+Rails applications include a `bin/ci` script that runs all essential checks for
+your app: setup, code style (RuboCop), security audits, and tests. The steps
+are defined in `config/ci.rb` and can be customized for your project.
 
-When we push our code to a GitHub repository with GitHub Actions enabled, it
-will automatically run these steps and report back success or failure for each.
-This allows us to monitor our code changes for defects and issues and ensure
-consistent quality for our work.
+This script prints each step as it runs, showing ✅ for success and ❌ for
+failures. If any step fails, `bin/ci` exits with a nonzero status.
+
+To use a CI Provider, point your pipeline to `bin/ci`. This ensures consistent
+checks locally and in CI.
+
+To run it locally, call the script from your command line:
+
+```bash
+$ bin/ci
+Continuous Integration
+Running tests, style checks, and security audits
+
+
+Setup
+bin/setup --skip-server
+
+== Installing dependencies ==
+The Gemfile's dependencies are satisfied
+
+== Preparing database ==
+
+== Removing old logs and tempfiles ==
+
+✅ Setup passed in 2.11s
+
+
+Style: Ruby
+bin/rubocop
+
+Inspecting 25 files
+.........................
+
+25 files inspected, no offenses detected
+
+✅ Style: Ruby passed in 1.17s
+# ...
+✅ Continuous Integration passed in 8.91s
+```
+
+### CI Steps DSL
+
+The file is written in a DSL that makes it straightforward to manage steps, here
+we add another step to check to make sure we don't leave any TODOs behind in
+the code in `config/ci.rb`:
+
+```ruby#6-7
+# config/ci.rb
+CI.run do
+  step "Setup", "bin/setup --skip-server"
+
+  step "Style: Ruby", "bin/rubocop"
+  step "Check: No TODOs",
+        "if grep -r TODO app/; then exit 1; fi"
+  # ...
+end
+```
+
+To test this out, I added a todo comment at the start of a random file, here
+the `ApplicationController`:
+
+```ruby
+# app/controllers/application_controller.rb
+# TODO: Remove this todo
+class ApplicationController < ActionController::Base
+# ...
+```
+
+Now when I run it on the CI:
+
+```bash
+$ bin/ci
+# ...
+Check: No TODOs
+if grep -r TODO app/; then exit 1; fi
+
+app/controllers/application_controller.rb:# TODO: Remove this todo
+
+❌ Check: No TODOs failed in 0.01s
+# ...
+❌ Continuous Integration failed in 9.09s
+```
+
+To verify, remove the comment:
+
+```ruby
+# app/controllers/application_controller.rb
+class ApplicationController < ActionController::Base
+# ...
+```
+
+and `bin/ci` should now pass:
+
+```bash
+$ bin/ci
+# ...
+Check: No TODOs
+if grep -r TODO app/; then exit 1; fi
+
+
+✅ Check: No TODOs passed in 0.01s
+# ...
+✅ Continuous Integration passed in 8.91s
+```
+
+To learn more about the DSL, read the documentation for [ActiveSupport::ContinuousIntegration](https://www.rubydoc.info/github/rails/rails/main/ActiveSupport/ContinuousIntegration).
 
 Deploying to Production
 -----------------------