Docs: refactor Pages intro guide

Type of issue

Documentation improvement.

Problem to solve

Remove redundant information on https://docs.gitlab.com/ce/user/project/pages/introduction.html.

Further details

This was the 1st doc on GitLab Pages, which has some general information about the feature.

We have created individual guides for each of Pages capabilities but did not remove the info from the 1st doc:

• Overview and get started: https://docs.gitlab.com/ce/user/project/pages/
• SSGs: https://docs.gitlab.com/ce/user/project/pages/getting_started_part_one.html
• Projects and URL structure: https://docs.gitlab.com/ce/user/project/pages/getting_started_part_two.html
• Custom domains + SSL/TLS: https://docs.gitlab.com/ce/user/project/pages/getting_started_part_three.html
• CI/CD for Pages: https://docs.gitlab.com/ce/user/project/pages/getting_started_part_four.html

Proposal

• Remove redundant info
• Move any info documented only there to other scopes documented elsewhere in the other docs

MRs:

• Pages part 1: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27136

Who can address the issue

@marcia

Other links/references

• For https://gitlab.com/gitlab-org/gitlab-ce/issues/39978