[TODO]: Replace mailserver.env in favor of improved ENV docs

[polarathene]

Description

As discussed here. mailserver.env and the ENV docs aren't entirely in sync, and it adds a maintenance burden (both in maintaining it and noise it often contributes to bug reports that copy/paste the whole thing instead of only sharing what was explicitly changed).

We could convey most value with restructuring of the ENV docs, but the primary improvements for referencing/overview of ENV would be:

• A ENV overview table at the top with key/value columns. The value being the default.
  • A third column could technically list alternative values (enum variants), or a description of the value expected.
  • Key column would list each ENV with a link to the more detailed section on that page for the ENV to get more information.
• If using Content Tabs feature, or a collapsed admonition; we could also include a .env / compose.yaml of the ENV with empty values for easy copy/paste if that is considered helpful to users.

This would better centralize our ENV (and docs for it). Especially since the docs are better versioned, we've had users try the master branch mailserver.env instead of from a tagged release that matches their image version (although still likely to happen with the latest docs we direct users to by default 🤷‍♂️ ).