ri: add page and link from ruby (#21451)

pages/common/ri.md

@@ -0,0 +1,17 @@
+# ri
+
+> Browse structured API documentation for Ruby.
+> See also: `ruby`.
+> More information: <https://ruby.github.io/rdoc/RI_md.html>.
+
+- Start interactive shell:
+
+`ri`
+
+- View documentation for a particular name:
+
+`ri {{File#read}}`
+
+- List classes and modules for which `ri` has documentation:
+
+`ri {{[-l|--list]}}`

pages/common/ruby.md

@@ -1,7 +1,7 @@
 # ruby
 
 > Ruby programming language interpreter.
-> See also: `gem`, `bundler`, `rake`, `irb`.
+> See also: `gem`, `bundler`, `rake`, `irb`, `ri`.
 > More information: <https://manned.org/ruby>.
 
 - Execute a Ruby script: