Docs: add ENV Vars

[patrickskowronekdkfz]

Like the CCP-Explorer, let's have a table about the environment variables you could use

[timakro]

We are a JS library running exclusively in the browser. So no env vars...

Would love it if we could accept some env vars in lens that would then work in every lens-based app but sadly we can't

[patrickskowronekdkfz]

You're absolutely right! I'll phrase it as a question instead.

@timakro , would it make sense to recommend using environment variables for deployment, as we do with the CCP Explorer? That way, we could simply document it.

[timakro]

I would like to see some standardized variables in the new app guide, e.g. PUBLIC_BACKEND_URL.