[docs-only] Add a full description about creating, using, naming, deprecating... envvars

We miss in the dev docs a section that fully describes how envvars are

created/implementd, used, named, (deprecated), differenciated against yaml etc.

For yaml, we have to sometimes add a description in the readme to make things clear.

This also includes a reference which types we currently have and why (there is a description already available in the admin docs we can reuse/reference). Which rules apply for a description ect.

This is an important info as we (I) again and again stumble over the same things and advice colleagues because they or those who want to contribute just dont know.