Docs: improve Pages overview (to deprecate feature page)

What does this MR do?

• Make GitLab Page's index page more "enticing" to deprecate the feature page https://about.gitlab.com/features/pages/
• Selectively bring in any useful visuals/information from the existing Feature page on about.gitlab if one exists
• Good summary/intro
• Selectively add any good YouTube video and/or blog link we may already have
• Easy-to-understand diagrams, videos, etc.

Review app: http://review-docs-docs-ndbq74.178.62.207.141.xip.io/ce/user/project/pages/index.html

Related issues

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50507
• For &305 (closed)

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/7509

Review checklist

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

Once merged

• Update features.yml
• Deprecate https://about.gitlab.com/features/pages/ (delete content, set up a redirect to docs)