FEATURE: Improved documentation of app config env vars

[brassy-endomorph]

Is your feature request related to a problem? Please describe.

As an aleph admin, I don't want to have to look through source code to figure out how to configure my aleph instance.

Describe the solution you'd like

Full documentation of every env var that can be set to change the behavior of the app (excluding vars like LC_*).

E.g.,

Name	Flask Equivalent	Required	Default	Explanation
ALEPH_DATABASE_URI	SQLALCHEMY_DATABASE_URI	yes		The connection string to the Postgres database
ALEPH_APP_TITLE		no	Aleph	The name of the app in the UI

Relates to #2437

[brassy-endomorph]

This was the direct cause of #2428