Refactor CI/CD landing page

What does this MR do?

To make it easier to navigate into the ci section of the documentation, this MR restructures the existing landing page.

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9412.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56973 and https://gitlab.com/gitlab-org/gitlab-ce/issues/50498.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9412

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9412), CE-MR afterwards