Mention `rails dev:cache` in Caching with Rails guide
Mention `rails dev:cache` in Caching with Rails guide

On a new Rails app, [`config/development.rb`](https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt#L19) is set up to support [`rails dev:cache`](https://github.com/rails/rails/pull/20961).

This PR explains how to enable caching in development in https://guides.rubyonrails.org/caching_with_rails.html, replacing the old advice that looks like it came before https://github.com/rails/rails/pull/20961.
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