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

[josemontespg]

As discussed in PR #1497 (#1497 (comment)), 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.