Add image input examples to Haystack docs for Chat Generators

[anakin87]

Currently, we support image inputs in many of our Chat Generators, but relative examples don't appear in Haystack Documentation website. We should fill this gap by adding code snippets.

Below, I am listing all required updates. Done means: "snippet added to the docs website".

If a snippet is already linked, we can reuse it.
Otherwise, I recommend trying out the Chat Generator locally and adding the code snippet as a comment to this issue, which can then be copied into the docs.

• OpenAIChatGenerator – snippet already added to docs
• HuggingFaceAPIChatGenerator – snippet
• AmazonBedrockChatGenerator – snippet
• AnthropicChatGenerator – snippet
• CohereChatGenerator – snippet
• LlamaCppChatGenerator – snippet
• WatsonxChatGenerator – snippet
• AzureOpenAIChatGenerator – no snippet yet
• MetaLlamaChatGenerator – no snippet yet
• MistralChatGenerator – no snippet yet
• NvidiaChatGenerator – no snippet yet
• OpenRouterChatGenerator – no snippet yet
• STACKITChatGenerator – no snippet yet
• GoogleGenAIChatGenerator – no snippet yet
• OllamaChatGenerator – no snippet yet