✨ [open-zaak/open-zaak#1649] Command to document envvars

`decouple.config` is overloaded to accept help text and several other attributes, as well as to add the loaded environment variables to a registry. This registry is used by the `generate_envvar_docs` management command to generate RST style documentation based on a template. This removes the need for duplication for these envvar docs in all the components that use open-api-framework

MANIFEST.in

@@ -2,6 +2,7 @@ include *.rst
 include LICENSE
 include open_api_framework/py.typed
 recursive-include open_api_framework *.html
+recursive-include open_api_framework *.rst
 recursive-include open_api_framework *.txt
 recursive-include open_api_framework *.po
 recursive-include open_api_framework/static *.css