docs(readme): fix links and imports, document all model providers (#837)

Author: awsarron

README.md

@@ -129,7 +129,6 @@ from strands import Agent
 from strands.models import BedrockModel
 from strands.models.ollama import OllamaModel
 from strands.models.llamaapi import LlamaAPIModel
-from strands.models.llamacpp import LlamaCppModel
 
 # Bedrock
 bedrock_model = BedrockModel(
@@ -159,12 +158,15 @@ response = agent("Tell me about Agentic AI")
 Built-in providers:
  - [Amazon Bedrock](https://strandsagents.com/latest/user-guide/concepts/model-providers/amazon-bedrock/)
  - [Anthropic](https://strandsagents.com/latest/user-guide/concepts/model-providers/anthropic/)
+ - [Cohere](https://strandsagents.com/latest/user-guide/concepts/model-providers/cohere/)
  - [LiteLLM](https://strandsagents.com/latest/user-guide/concepts/model-providers/litellm/)
  - [llama.cpp](https://strandsagents.com/latest/user-guide/concepts/model-providers/llamacpp/)
  - [LlamaAPI](https://strandsagents.com/latest/user-guide/concepts/model-providers/llamaapi/)
+ - [MistralAI](https://strandsagents.com/latest/user-guide/concepts/model-providers/mistral/)
  - [Ollama](https://strandsagents.com/latest/user-guide/concepts/model-providers/ollama/)
  - [OpenAI](https://strandsagents.com/latest/user-guide/concepts/model-providers/openai/)
- - [Writer](https://strandsagents.com/latest/documentation/docs/user-guide/concepts/model-providers/writer/)
+ - [SageMaker](https://strandsagents.com/latest/user-guide/concepts/model-providers/sagemaker/)
+ - [Writer](https://strandsagents.com/latest/user-guide/concepts/model-providers/writer/)
 
 Custom providers can be implemented using [Custom Providers](https://strandsagents.com/latest/user-guide/concepts/model-providers/custom_model_provider/)