Review and Overhaul CI/CD Docs
## Background
CI/CD docs should:
- Be much easier to approach and navigate
- Be more useful
- Offer a clear picture of the capabilities and how they fit into the rest of the GitLab workflow.
- Include goal-oriented use case guides.
Following https://gitlab.com/gitlab-org/gitlab-ce/issues/50498, and prioritizing the main index page, let's review this whole section for **accuracy, completeness, and flow/organization**, taking into account the following, and further needs encountered in a discovery process.
## Completed Work
The following work has been completed:
- [X] CI/CD landing page overhauled in issue (https://gitlab.com/gitlab-org/gitlab-ce/issues/56973 ) and merge request (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24894).
- [X] Ensure complete CI/CD reference page, and restructure it in issue (https://gitlab.com/gitlab-org/gitlab-ce/issues/51675) and merge request (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25444).
- [X] Collect and evaluate existing resources outside of main documentation, and make more discoverable and navigable in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58041 and merge request https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25588.
- [X] Restructure Pipelines page around functional areas (visualising, configuring, using) in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58450 and merge request https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26079.
- [X] Add more introductory material in CI/CD documentation in merge request https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25611.
- [X] Tweak the navigation of CI/CD in the global docs navigation in merge requests https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/426 and https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/429.
- [X] New diagram to explain CI/CD in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58720 and merge request https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25934.
- [X] Reorganise and refactor CI Variables documentation in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58626 and merge request https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26336.
- [X] Basic CI/CD workflow illustration (2nd iteration): https://gitlab.com/gitlab-org/gitlab-ce/issues/59108, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26733
## In Progress Work
- [x] Gather all configuration-related documentation together in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58451. (Evan)
- [x] Diagrams for CI/CD: https://gitlab.com/gitlab-org/gitlab-ce/issues/59110 => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27278. (Marcia)
- [x] Ensure the index page is further streamlined / easy to approach, and fully meets feedback provided in @markpundsack feedback in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58323: [Doc](https://docs.google.com/document/d/13hpZOP2D5EWLOjpSAZh-wXZoCDO7iaDIBNdx_8y1Qzc/edit) and [video](https://drive.google.com/file/d/1XJNcmppkZNDSNmxJHuvLtJ3TlrMFqCfc/view?usp=sharing). => https://gitlab.com/gitlab-org/gitlab-ce/issues/58323 // https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27620 (Marcia)
## Scheduled Work
The following are identified needs that are still to be addressed:
- [x] Continue to work on getting started material in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/38964 and https://gitlab.com/gitlab-org/gitlab-ce/issues/58352.
- [x] Complete stock take of all relevant CI/CD documentation in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58326.
- [ ] Add in-depth guides for key workflows in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58042
- [x] Add tools migration docs in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/58044.
- [ ] Improve parallel execution docs in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/54459.
- [x] Explain that we use codeclimate for code quality in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/54469.
- [x] Review CI docs content/structure: https://gitlab.com/gitlab-org/gitlab-ce/issues/58481#content-pages
## Resources
The following are resources outside issues and MRs relevant to the work.
- CI/CD Doc Structure scratch pad: https://docs.google.com/document/d/13OzhhA-BqQtKN8QLu_Q4DrpT1s4gU74U6h9mD5jS7aQ/edit?usp=sharing.
- CI/CD Doc needs discussion: https://docs.google.com/document/d/1vkYxIUJ1uwbuw3MJIBVbNKyt5j5LFsFIhgVRsj9QiSE/edit?usp=sharing
- CI/CD Teardown [Doc](https://docs.google.com/document/d/13hpZOP2D5EWLOjpSAZh-wXZoCDO7iaDIBNdx_8y1Qzc/edit) and [video](https://drive.google.com/file/d/1XJNcmppkZNDSNmxJHuvLtJ3TlrMFqCfc/view?usp=sharing)
- Redit post on getting started guides: https://web.archive.org/web/20190128163711/https://www.reddit.com/r/devops/comments/ajkdro/how_to_apply_cicd_by_using_github_codebuild/eewnlx9/.
- Blog post on documentation types: https://www.divio.com/blog/documentation/.
- Slack discussion on Migration guides: https://gitlab.slack.com/archives/C16HYA2P5/p1549891187004100.
- Slack discussion on Getting started guides: https://gitlab.slack.com/archives/C1BBL1V3K/p1548693105307200?thread_ts=1548422475.278500&cid=C1BBL1V3K.
- Content files in docs analysis: https://docs.google.com/spreadsheets/d/1GjMIRBDwBcJe09RsUuDsFlwketS5p-kQgbuyBGYXMNM/
- Blog post: https://medium.com/@einenlum/a-free-ci-using-docker-compose-for-your-gitlab-repositories-f1289e059a77
- Articles: https://vadosware.io/categories/gitlab/
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