rails: add generate, db:migrate, and routes examples (#15674)

pages/common/rails.md

@@ -8,6 +8,18 @@
 
 `rails new "{{project_name}}"`
 
+- Generate a scaffold for a model named Post, predefining the attributes title and body:
+
+`rails generate scaffold Post title:string body:text`
+
+- Run migrations:
+
+`rails db:migrate`
+
+- List all routes:
+
+`rails routes`
+
 - Start local server for current project on port 3000:
 
 `rails server`