Docs: update Pages IP address change + other DNS updates

What does this MR do?

• Follow up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21163/
• Updates link to GCP migration blog post
• Update screenshot
• Fix headings
• Add note for CNAME apex
• Add subsection for redirecting www to domain.com when using CloudFlare (https://gitlab.com/gitlab-org/gitlab-ce/issues/48848#note_87314849)
• Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/39978

Review app: http://review-docs-docs-u3yoho.178.62.207.141.xip.io/ce/user/project/pages/getting_started_part_three.html

Related issues

• https://gitlab.com/gitlab-org/gitlab-ce/issues/47585
• https://gitlab.com/gitlab-org/gitlab-ce/issues/48848
• https://gitlab.com/gitlab-com/infrastructure/issues/4779
• gitlab-com/www-gitlab-com!13265 (merged)

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6918

Review checklist

• Your team's review (required) (@axil)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards

cc/ @nick.thomas @jlenny @mikelewis FYI