Docs: review/refactor Pages doc
Problem to solve
- Improve the doc overall for better reading flow: GitLab Pages custom domains and SSL/TLS Certificates
- Rename document to
pages_custom_domains_and_certificates.md - Move conceptual information elsewhere
- Isolate the step-by-step guides for adding the domain and the certicate
Further details
After multiple additions to this doc it's become too extense and requires the user to read all of that to achieve more straightforward tasks.