Developer Code Contributing Guidelines

### Summary
In order to ensure quality and consistency in our documentation, it would be beneficial to establish some guidelines when contributing code or functions to our products. By doing so, we can empower code contributors to contribute effectively.

Related with findings in https://github.com/insightsengineering/teal.widgets/issues/148#issuecomment-1489421749

### Several examples of guidelines
* Centralize `import` and `importFrom` statements in the `package.R` file instead of in each function file.
* https://r-pkgs.org/man.html#sec-man-package-doc
* Include a title and a description in the roxygen notes for every function.
* Exported functions and objects should have an `@example` section.
* If it is wrapped in a `\dontrun{}`, the example should still be runnable interactively.
* The appropriate scenario to use `import`, `importFrom`, or `::` when writing functions.

### Tasks
* Discuss with team
* Run a PoC
* Create a set of guidance that we can follow
* Implement to one package and assess
* https://github.com/insightsengineering/teal/issues/818
* https://github.com/insightsengineering/teal.widgets/issues/149
* https://github.com/insightsengineering/teal.transform/issues/128
* Finalize guidelines
* Make it public