Add GitLab Pages CI/CD

Attempt to fix #6 (closed) by using GitLab Pages (CI/CD). References:

• https://gitlab.com/pages/hugo
• https://docs.gitlab.com/ee/user/project/pages/
• https://docs.gitlab.com/ee/user/project/pages/getting_started/pages_from_scratch.html

Removing use of blackfriday markdown handler because "Blackfriday was removed in Hugo v0.100.0." (build failure in https://gitlab.com/ssbc/scuttlebutt.nz/-/jobs/3574989513); using default goldmark instead (https://gohugo.io/getting-started/configuration-markup/#goldmark); but continuing to use blackfriday for heading ids (autoHeadingIDType) to preserve existing ids (anchor links), e.g. this.

With this merged, pages deployment information should appear here: https://gitlab.com/ssbc/scuttlebutt.nz/pages? under gitlab.io according to https://docs.gitlab.com/ee/user/project/pages/getting_started_part_one.html#gitlab-pages-default-domain-names. Then we could see about making it use our custom domain (https://docs.gitlab.com/ee/user/project/pages/custom_domains_ssl_tls_certification/index.html#set-up-a-custom-domain).