developer: add a developer profile example

[rlahfa-dinum]

This adds an example on how to use the developer profile.

The rationale for not pursuing an approach by importing developer/ as needed is the following: the user inventory file has special requirements due to limitations in Sécurix's code (that can be lifted in exchange of more complexity inside the code). These requirements prevent us to write something like:

{ bureautixModulesPath, lib, ... }: {
   securix.self.user = { ... };
   imports = [ "${bureautixModulesPath}/developer" ];
}

which would have been an idiomatic way to some extent to do it.

Instead, we pick the path of least resistance (and also very simple to reason about at scale): a developer profile is always imported, disabled by default and gated behind the module system. User inventories can turn on the flag to become developers, this is what is done.

[jdauphant-dinum]

si je veux importer plusieurs fichiers pour faire la config dev, je vais devoir faire "config = mkIf config.bureautix.developer.enable {" dans chaque fichier ?

[rlahfa-dinum]

Oui, il faudra faire ça dans chaque fichier, il n'est pas possible d'importer conditionnellement des fichiers dans le module system.

Si on veut faire ça, on doit exposer un "méta flag" développeur et ça complexifera d'autres choses.

[rlahfa-dinum]

@jdauphant-dinum Changement requis, fait, je te laisse me dire si ça te convient.