Update readme to highlight alternative content generators

[jacobsimionato]

This PR updates the top-level README.md to better inform developers about the different ContentGenerator implementations available for connecting the GenUI framework to an AI backend.

Previously, the README only mentioned the genui_firebase_ai package, which could lead developers to believe that Firebase AI Logic is the only supported backend. This change clarifies that there are multiple options for different use cases.

Changes:

• Adds a new "Connecting to an AI Agent" section: This briefly introduces the
  ContentGenerator concept, which allows genui to be backend-agnostic.
• Updates the main "Packages" table: The table now includes
  genui_google_generative_ai and genui_a2ui and provides clear, concise
  descriptions for each package, highlighting the specific ContentGenerator it
  contains and its primary use case (e.g., production, prototyping, custom
  servers).

This makes it easier for developers to quickly understand and choose the right package for their project.

Fixes #522

[gemini-code-assist]

Code Review

This pull request does a great job of updating the main README to clarify the different ContentGenerator options available for connecting to an AI backend. The new 'Connecting to an AI Agent' section and the updated packages table make it much clearer for developers to choose the right implementation for their needs.

My review includes a couple of suggestions to further improve the clarity and readability of the documentation. I've recommended a minor formatting change to a paragraph for better readability and a wording adjustment in the packages table to make the use case for genui_google_generative_ai more explicit. Overall, these are excellent changes that will improve the developer experience.