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