Reference docs of environment variables and project config

[rcannood]

• Environment variables is currently listed under "Reference > Config > Environment Variables", but I think it doesn't make a lot of sense to place this under the config.
• One solution would be to add information regarding the environment variables in "Reference > CLI > Environment variables"
• Another solution would be to mention it in each of the documentation pages of the viash build, viash test, viash ns build...