Docs: Refactor Pages' intro guide

What does this MR do?

Refactors Pages intro guide, remove redundant info:

• Moving content from intro # Getting started with GitLab Pages domains, # User or group Pages, #Project Pages to their own doc (getting_started_part_one.md).
• Moving "Introduced in" notes to the beginning of Pages's docs.
• Leaving only content not documented elsewhere.

Next steps:

• Rename getting_started_part_one.md to pages_domains.md.
• Rename introduction.md to pages_settings.md? (TBD)
• Move other content to getting_started_part_*.md guides.

Related issues

For https://gitlab.com/gitlab-org/gitlab-ce/issues/58178, https://gitlab.com/gitlab-org/gitlab-ce/issues/39978.

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

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.