ruby, gem: update page; irb, rake: add page (#10412)

* gem: update page

* Use `snake_case` for placeholders
* Update the documentation URL

* ruby: move `irb` example to a separate page

* rake: add page

* ruby: reference other ruby-related pages

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

* irb: standard input => `stdin` (#10436)

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/irb.md

@@ -0,0 +1,9 @@
+# irb
+
+> Interactive Ruby shell.
+> Evaluate Ruby code read from `stdin`.
+> More information: <https://docs.ruby-lang.org/en/master/IRB.html>.
+
+- Start the interactive shell:
+
+`irb`