doc: Update documentation of GoogleAI and VertexAI to show streaming example

[Amnah199]

Is your feature request related to a problem? Please describe.
The below four components now support streaming_callback, similar to the OpenAIGenerator. After their release, it would be beneficial to include an example of 'Streaming' in the documentation to demonstrate its usage.

• VertexAIGeminiChatGenerator
• VertexAIGeminiGenerator
• GoogleAIGeminiChatGenerator
• GoogleAIGeminiGenerator

Describe the solution you'd like
Add a separate example code for streaming, similar to the example provided for the OpenAIGenerator in the documentation:
OpenAIGenerator Example.

[julian-risch]

Daria added a section about streaming to the docs of all generators that support streaming. For example this section https://docs.haystack.deepset.ai/docs/vertexaigeminichatgenerator#streaming points to https://docs.haystack.deepset.ai/docs/choosing-the-right-generator#streaming-support

And there is a new column in the overview for streaming support https://docs.haystack.deepset.ai/docs/generators

With all that being done, we can close this issue.