docs: Update and improve README examples of number/float64 namespace #1595

### Description

This RFC proposes enhancing the README examples for the number/float64 namespace package. Presently, the READMEs for namespace packages offer minimal examples that lack informative value and fail to showcase the functionality of the namespace adequately.

In the past, examples within namespace packages received limited attention due to the dynamic nature of the project's development. However, with the project's development and organization now stabilized, it's imperative to revisit these packages and incorporate comprehensive examples. These examples will serve to effectively communicate to users how to utilize the contents of the namespace

### Related Issues

None

### Questions

No.

### Other

No.

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `RFC:`.

Locate the README files for the number/float64 namespace package and review their existing examples and the namespace contents. Improve the examples so they communicate the package's functionality more effectively; done means the README examples are comprehensive and informative.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.