Docs: Introduction to continuous methods and GitLab CI/CD

What does this MR do?

• Introduces the concept of continuous methods: Continuous Integration, Continuous Deployment, and Continuous Delivery.
• Introduces how GitLab CI/CD works (super high-level overview)
• Is a first MVC for https://gitlab.com/gitlab-org/gitlab-ce/issues/38964 and https://gitlab.com/gitlab-org/gitlab-ce/issues/58394
• To be followed by (in further iterations):
  • A "hello world" guide which guides the user to use GitLab CI/CD for the 1st time => https://gitlab.com/gitlab-org/gitlab-ce/issues/58352
  • Add different docs with proposed workflows for teams using CI/CD (with actual code) => https://gitlab.com/gitlab-org/gitlab-ce/issues/58042
• Add the doc to a new folder ci/introduction/ where we can add more introductory docs, guides, walkthroughs.

REVIEW APPS:

• New doc: http://review-docs-docs-rcuo6b.178.62.207.141.xip.io/ce/ci/introduction/
• CI/CD landing page: http://review-docs-docs-rcuo6b.178.62.207.141.xip.io/ce/ci/

Related issues

• https://gitlab.com/gitlab-org/gitlab-ce/issues/38964
• https://gitlab.com/gitlab-org/gitlab-ce/issues/38970
• &784 (closed)
• &548
• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58394

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page.
• Apply the ~Documentation label.
• Set the milestone.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.
4. Merge EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9826), this one later.
5. For myself: add to the global nav: https://gitlab.com/gitlab-com/gitlab-docs/issues/348