Docs: how to set up Let's Encrypt for GitLab Pages websites

What needs a new doc

• Document GitLab Pages custom domains + Let's Encrypt
• Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21798#note_104934089
• Re-write an up-to-date tutorial to replace the old blog post
• Update the link from the Pages index doc

New doc or update?

• New documentation

Checklists

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature: ce/user/project/pages/
• Fill the doc blurb below

Documentation blurb

• Doc title

  How to secure GitLab Pages websites with Let's Encrypt

• Feature overview/description

  TBA

• Feature use cases

  TBA

Author of the doc

• Copy the doc blurb above and paste it into the doc
• Write the tutorial - explain how to use the feature
• Submit the MR using the appropriate MR description template

ETA

As soon as possible, given our priorities. I'll tentavely add %11.5