[ruby/irb] Use the documentation site as the source of truth

(ruby/irb#1055)

* Use the documentation site as the source of truth

1. Remove detailed content from README.md and point links to the documentation site.
2. Remove the content of EXTEND_IRB.md and point links to the documentation site.

* Use GitHub pages as Rubygems' documentation target

ruby/irb@d2b73cb91e

Author: st0012

lib/irb/irb.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |spec|
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
-  spec.metadata["documentation_uri"] = spec.homepage
+  spec.metadata["documentation_uri"] = "https://ruby.github.io/irb/"
   spec.metadata["changelog_uri"] = "#{spec.homepage}/releases"
 
   spec.files         = [