rusk-wallet: Document (custom) multi-network configurations

[ZER0]

Describe what you want documented
Now the file config.toml is totally different, and it can also accept alternative networks. This feature is not well documented yet, and it should.

Describe "Why" this is needed
To let the user understand how to configure the wallet's network – however the default values are the one used the most.

Describe alternatives you've considered
N/A

Additional context
Since now the application contains three different level of configuration (default, global, profile) we might also change the behavior so that when it runs, if no config.toml is found on the profile folder, and no is found on the global folder, it might create one config.toml file with the default settings in the global one – this is a better approach compare to the previous one where global didn't exists, and therefore the configuration was written in the profile.

In doing so, we might add comments directly to the config.toml too, explaining how it works. This is a common practice for app that relies on such configuration files.