Renderer Guide: Consider using `formatNumber` instead of `formatString` for initial basic catalog support

As discussed in PR #1497 (https://github.com/a2ui-project/a2ui/pull/1497#discussion_r3416494545), we should consider using `formatNumber` instead of `formatString` in the initial bootstrap steps of the `renderer_guide.md`.

Currently, the guide suggests implementing `formatString`, which requires relatively complex string expression parsing in the core library (e.g., parsing expressions like `Some string then ${myFunction(someNestedFunction(/someDataModelValue))}`). Using `formatNumber` instead would allow implementing the smallest possible subset of the basic catalog for meaningful end-to-end testing.

This change will also require updating the `00_` scaffolding examples to use `formatNumber` instead of `formatString`.

Look at the renderer_guide.md file and the 00_ scaffolding examples. Find where formatString is currently used and replace it with formatNumber. Verify the changes by checking if the basic catalog still works for end-to-end testing.

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