Document environments and deployments

## What does this MR do?

Adds an introduction to environments and deployments and tweaks the `environments` entry in `.gitlab-ci.yml` documentation.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## What are the relevant issue numbers?

Closes #18683.

## Screenshots (if relevant)

![image](/uploads/1e3355c543abaf7b358f3c37596b1f0b/image.png)

![image](/uploads/58a51505674bcf605039db46833a8ae6/image.png)

/cc @ayufan @axil 

See merge request !4725