Remove link to Cloudflare cert docs from GitLab Pages docs

Problem to solve

"Cloudflare certificates" docs directly linked from GitLab Pages' "Getting Started" section is now not mandatory for everyone who uses GitLab Pages with HTTPS as GitLab equips Let's Encrypt integration with Pages since 12.1, released on 2019-07-22.

So we can safely remove a link to Cloudflare cert docs from Getting Started section of GitLab Pages docs. A link to that page can be added to the doc for manually adding SSL/TLS certificates to GitLab Pages