Add foreman/overmind requirement note to Getting Started guide

New developers following the Getting Started guide encounter the "can't
find executable foreman" error when running ./bin/dev, but this
requirement is only documented in the "Installation into an existing
Rails app" guide.

This commit adds a clear note in the "Start the app" section to ensure
developers know they need overmind or foreman installed before running
bin/dev, improving the getting started experience.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

docs/getting-started.md

@@ -71,9 +71,11 @@ Start the app:
 ```bash
 bin/dev help
 bin/dev # start with hmr
-bin/dev static #
+bin/dev static # start with statically created bundles (no HMR)
 ```
 
+> **Note:** Ensure that you have `overmind` or `foreman` installed to run `bin/dev`.
+
 ## Basic Usage
 
 ### Configuration