Better document triggers docs

## What does this MR do?

- Refactor the triggers page view
- Replace old images with new ones
- Mention triggers in `ci/yaml/README.md`

## Screenshots (if relevant)

The new triggers views contains more information and is prettier :)

## Before

![screenshot-localhost_3000_2016-09-06_10-53-10](/uploads/8f45a0e21a31f1819af1739f7af8afd0/screenshot-localhost_3000_2016-09-06_10-53-10.png)

# After

![screenshot-localhost_3000_2016-09-06_10-36-34](/uploads/2bbe2801b0f959851a8cfa5a8a14623d/screenshot-localhost_3000_2016-09-06_10-36-34.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21773

See merge request !6217