✨[#64] add subgroup to generate_envvar_docs

[Coperh]

Fixes #64

[Coperh]

Do you think this is fine or should I try to implement the automatic help text retriever from setup config?

Not sure how this would be possible since envvars are not linked to the setup steps.
Maybe iterate through each step and each variable and check if that variable is found in the envvars.

[stevenbal]

Looks good, tested it briefly with Open Klant and it works.

The only thing I'm not sure about is how we fit in the general documentation about setup configuration. The current docs for OZ for example describe how the command can be used and also has one or two sentences to describe what each step does: https://github.com/open-zaak/open-zaak/blob/main/docs/installation/config/openzaak_config_cli.rst#open-zaak-configuration-cli.

I'm not sure if grouping setup-configuration envvars together with all the other environment vars is the right way to go, also because the former are more of a one time thing to set things up, while the "regular" envvars can be modified to directly change configuration.

@annashamray what do you think?