* Adds pipelines page to CI docs.
* Adds image of pipelines list (although currently missing border that is on other images).
* Changes CI to CI/CD in `/doc` and `/doc/ci`
* Sorts user documentation in `/doc`
Partially fixes#17733.
See merge request !4660
Signed-off-by: Rémy Coutable <remy@rymai.me>
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)


/cc @ayufan @axil
See merge request !4725
Link to the gitlab-ci.yml file of GitLab itself.
I'm hunting for this link from time to time in customer demo's so I think it is useful to add.
/cc @ayufan
See merge request !1993