Strip unused metadata from pages

Remove metadata items we are not using now. They can be re-added, if
needed, in the future.

doc/user/project/pages/custom_domains_ssl_tls_certification/index.md

 ---
-last_updated: 2020-07-25
-type: reference, howto
 disqus_identifier: 'https://docs.gitlab.com/ee/user/project/pages/getting_started_part_three.html'
 stage: Release
 group: Release Management

doc/user/project/pages/getting_started/pages_from_scratch.md

 ---
-last_updated: 2020-01-06
-type: reference, howto
 stage: Release
 group: Release Management
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers

doc/user/project/pages/getting_started_part_one.md

 ---
-last_updated: 2018-06-04
-type: concepts, reference
 stage: Release
 group: Release Management
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers

doc/user/project/pages/introduction.md

 ---
-type: reference
-last_updated: 2020-01-06
 stage: Release
 group: Release Management
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers

doc/user/project/pages/lets_encrypt_for_gitlab_pages.md

 ---
 description: "How to secure GitLab Pages websites with Let's Encrypt (manual process, deprecated)."
-type: howto
-last_updated: 2019-07-15
 ---
 
 # Let's Encrypt for GitLab Pages (manual process, deprecated)