docs: update mcp_calculator README to reflect Pong game sample transi…

[sugoi-yuzuru]

…tion and summoning methods

Description

Update the readme to focus on helping developers test the Pong game example in its 3 different forms: MCP App, Iframe via URL, Iframe via srcdoc

Pre-launch Checklist

One time:

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have built and run at least one sample.

For this PR:

• I have updated the relevant CHANGELOG.md file.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on a fork, I have verified that scripts/e2e_test.sh passes.

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

[gemini-code-assist]

Code Review

This pull request updates the README.md file for the Angular client sample, transitioning it from describing the MCP Calculator to the Pong game sample. The updated documentation outlines the prerequisites, setup, and various summoning methods for the Pong game. The review feedback suggests improving the clarity of the setup instructions by explicitly adding directory navigation commands (cd) inside the code blocks for installing dependencies and starting the Angular application.