Mention the new generative ai content generator in the README, so folks know they don't NEED firebase ai logic

[sethladd]

[sethladd]

@jacobsimionato or @gspencergoog , can one of you help take care of this?

we want to communicate a few things in the README:

• there are multiple options for content generators (which we do, at a table towards the bottom)
• when to use each content generator
• there is a google_generative_ai generator
• there is a a2ui generator

[gspencergoog]

Okay, how about we move the table to the top, separate out their usage by generator, and include the google_generative_ai and a2ui generators in the lists.

[jacobsimionato]

Coding agent prompt:
Update the top-level README to mention the google_generative_ai_content_generator and a2ui_content_generator so that developers know they don't need to use Firebase AI Logic.

Consider:

• Explain each of the content generators and what they are used for
• The packages that each content generator is implemented in
• Consider also filling out the table of packages which already exists at the bottom of the page. This can probably document each content generator, but there should probably be some other sentence or paragraph that explains what a content generator is and the idea that there are multiple you can use.

Here is some information on the content generators:

`genui` provides three `ContentGenerator` implementations for connecting to
different types of agent providers. Use the following guide to choose the one
that best fits your needs.

- **`FirebaseAiContentGenerator`**: Use this when you are ready to deploy your
  app. This generator connects to Gemini via Firebase AI Logic, and is the
  recommended approach for production apps.

- **`GoogleGenerativeAiContentGenerator`**: Use this for prototyping and local
  development. This generator connects to the Google Generative AI API using an
  API key which is fast to set up.

- **`A2uiContentGenerator`**: Use this to connect to a server that implements
  the A2UI protocol. This is a good choice if you have an agent server that
  you want to connect to `genui`.

You should read some of the code and readmes for each package to write this correctly.