docs: Replace agent-and-renderer.png by a Mermaid diagram.

[ditman]

Description

There's a diagram in the glossary that doesn't look that great:

I replaced it by the following Mermaid diagram:

(Mermaid is already used in the "Renderer Stack" section of the glossary)

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

If you need help, consider asking for advice on the discussion board.

[ditman]

@polina-c is there any reason why the Glossary page is not linked from the left-hand side menu of the page? I think it'd be a good fit under the "Concepts" section? (I can modify the mkdocs.yaml in this PR to add it, so it's easier to find!)

[gemini-code-assist]

Code Review

This pull request replaces a static image in the glossary documentation with a Mermaid sequence diagram to illustrate the A2UI protocol flow. The reviewer suggested refining the diagram's loop name and adding an explicit note for data updates to better align with the textual description.