Classic Doom secret: walk backwards, get cool weapon

I am trying to remember a secret in old (classic?) Doom game. Point is that you start level on a platform/balcony. If player walks forward towards monsters, you'll jump on the floor and won't be able to get on platform again. However, if you turn around, there is a not-so-hidden passage to a room with chainsaw or something else.

What was the name of that Doom version and what was the level number, as well as the weapon?

Spoiler alert: I remember that episode while watching "Ready Player One".

There is a secret, where players should basically drive in reverse to solve.