Enhance GitLab CI/CD index page from feedback

Problem to solve

Further refinement of Gitlab CI/CD index page is necessary.

Further details

Improve CI/CD index page using feedback from Mark P:

• Doc
• Video

Proposal

• Improve https://docs.gitlab.com/ee/ci/README.html
• Make it easier to approach. Too many subsections -- it's hard to know where to go, as a reader.

Implementing MRs

• Modify navigation menu: https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/426

Other links/references