rails: add page (#1696)

pages/common/rails.md

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+# rails
+
+> A server-side MVC framework written in Ruby.
+
+- Create a new rails project:
+
+`rails new "{{project_name}}"`
+
+- Start local server for current project on port 3000:
+
+`rails server`
+
+- Start local server for current project on a specified port:
+
+`rails server -p "{{port}}"`
+
+- Open console to interact with application from command line:
+
+`rails console`
+
+- Check current version of rails:
+
+`rails --version`