feat: Explain how to use the flat file with internal functions

[statnmap]

As dev, I would like to be able to use sections to separate sub-functions, but I do not want these sections to appear in the vignette.

Example of use

• How does the example below works when inflated ?
• Does the section, even commented, is recognized as a title by {parsermd} ?
• How does it look like in the vignette ?

# One title for both groups of chunk

The code and tests for the main function

```{r function-fun1}`r ''`
```

```{r example-fun1}`r ''`
```

```{r test-fun1}`r ''`
```

<!--
# A new section for the code and tests for a sub-function that is not exported, 
 for which the example should be dontrun, 
-->

```{r function-fun2}`r ''`
```

<!-- If the function is @noRd, the example here will not be run in examples of the check.
 It should be in dontrun{} though
Also, another possibility is to name it `dev` instead of `example`
=> Try with -1- @noRd, but no dontrun{}, -2- both, -3- rename to `dev` 
-->

```{r example-fun2}`r ''`
```

```{r test-fun2}`r ''`
```