[rb] Adding doc link

Credit to @jjb

Closes #14897

Author: diemol

rb/selenium-webdriver.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.homepage = 'https://selenium.dev'
   s.metadata = {
     'changelog_uri' => 'https://github.com/SeleniumHQ/selenium/blob/trunk/rb/CHANGES',
+    "documentation_uri" => "https://www.selenium.dev/documentation/?tab=ruby",
     'github_repo' => 'ssh://github.com/SeleniumHQ/selenium',
     'source_code_uri' => 'https://github.com/SeleniumHQ/selenium/tree/trunk/rb',
     'rubygems_mfa_required' => 'true',