Exclude irb from documentation generation

- IRB has its own documentation in the project readme and https://ruby.github.io/irb/ already.
- As a tool, its internal implementation details are not relevant to the user.
- Excluding it from the documentation generation reduces the size of the generated documentation
  and makes search results more relevant.

I also checked the there are no references to IRB so it should not break any links.

.rdoc_options

@@ -7,3 +7,6 @@ title: Documentation for Ruby development version
 visibility: :private
 rdoc_include:
 - doc
+
+exclude:
+- lib/irb

lib/irb/.document

@@ -1 +0,0 @@
-**/*.rb