docs: split out postage stamps docs

[AuHau]

We need to clean up a bit of the README. Generally, the Postage Stamps management is now a very big feature of gateway-proxy, so we should split it from all the rest of the small features to make it a bit more structured.

• can we split the "Environment variables" section into at least two sections, where one would contain general variables and one all the postage stamp's management

---- to be finished ---