Docs: .gitlab-ci.yml getting started - improve voice and tone

Problem to solve

The language used in https://docs.gitlab.com/ee/user/project/pages/getting_started_part_four.html is a little bit too "bloggy". Let's make it shorter and more direct and straightforward.

Further details

Maybe also move the content directly into ci/ and link Pages from there? (TBD)

Proposal

Revamp the doc.

Who can address the issue

@marcia - %"Next 3-4 releases" (to be scheduled)

Other links/references

Related to:

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