Add Cohere API key config to documentation

Author: infinityrobot

docs/configuration.md

@@ -50,6 +50,7 @@ RubyLLM.configure do |config|
   config.anthropic_api_key = ENV.fetch('ANTHROPIC_API_KEY', nil)
   config.gemini_api_key = ENV.fetch('GEMINI_API_KEY', nil)
   config.deepseek_api_key = ENV.fetch('DEEPSEEK_API_KEY', nil)
+  config.cohere_api_key = ENV.fetch('COHERE_API_KEY', nil)
   config.openrouter_api_key = ENV.fetch('OPENROUTER_API_KEY', nil)
   config.ollama_api_base = ENV.fetch('OLLAMA_API_BASE', nil)
 
@@ -104,6 +105,7 @@ Set the corresponding `*_api_key` attribute for each provider you want to enable
 *   `anthropic_api_key`
 *   `gemini_api_key`
 *   `deepseek_api_key`
+*   `cohere_api_key`
 *   `openrouter_api_key`
 *   `ollama_api_base`
 *   `bedrock_api_key`, `bedrock_secret_key`, `bedrock_region`, `bedrock_session_token` (See AWS documentation for standard credential methods if not set explicitly).

docs/guides/rails.md

@@ -150,6 +150,7 @@ RubyLLM.configure do |config|
   # Add other provider configurations as needed
   config.anthropic_api_key = ENV['ANTHROPIC_API_KEY']
   config.gemini_api_key = ENV['GEMINI_API_KEY']
+  config.cohere_api_key = ENV['COHERE_API_KEY']
   # ...
 end
 ```