doc: Add an example sinatra app using openfeature-sdk

examples/Gemfile

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+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gem "concurrent-ruby"
+gem "cowsay"
+gem "openfeature-sdk", path: ".."
+gem "puma"
+gem "sinatra"

examples/Gemfile.lock

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+PATH
+  remote: ..
+  specs:
+    openfeature-sdk (0.1.0)
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    concurrent-ruby (1.2.2)
+    cowsay (0.3.0)
+    mustermann (3.0.0)
+      ruby2_keywords (~> 0.0.1)
+    nio4r (2.5.9)
+    puma (6.3.1)
+      nio4r (~> 2.0)
+    rack (2.2.8)
+    rack-protection (3.1.0)
+      rack (~> 2.2, >= 2.2.4)
+    ruby2_keywords (0.0.5)
+    sinatra (3.1.0)
+      mustermann (~> 3.0)
+      rack (~> 2.2, >= 2.2.4)
+      rack-protection (= 3.1.0)
+      tilt (~> 2.0)
+    tilt (2.2.0)
+
+PLATFORMS
+  arm64-darwin-22
+
+DEPENDENCIES
+  concurrent-ruby
+  cowsay
+  openfeature-sdk!
+  puma
+  sinatra
+
+BUNDLED WITH
+   2.4.19

examples/app.rb

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+# frozen_string_literal: true
+
+require "sinatra"
+require "cowsay"
+require "openfeature/sdk"
+
+OpenFeature::SDK.configure do |config|
+  config.provider = OpenFeature::SDK::Provider::NoOpProvider.new
+end
+feature_flags = OpenFeature::SDK.build_client(name: "my-app")
+
+get "/" do
+  if feature_flags.fetch_boolean_value(flag_key: "with-cows", default_value: false)
+    "<pre>#{Cowsay.say "Hello, world!", "cow"}</pre>"
+  else
+    "<p>Hello, world!</p>"
+  end
+end