Add docs link to RubyGems metadata (#458)

Author: andyw8

ruby-lsp-rails.gemspec

@@ -16,6 +16,7 @@ Gem::Specification.new do |spec|
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
   spec.metadata["changelog_uri"] = "#{spec.homepage}/releases"
+  spec.metadata["documentation_uri"] = "https://shopify.github.io/ruby-lsp/rails-addon.html"
 
   spec.required_ruby_version = ">= 3.0.0"