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`.