FEAT: Automatic site and PDF publication

added GitHub Actions publication workflow script to .github/workflows/jekyll-gh-pages.yml;
added spec-publisher project as a git submodule;
using basic Markdown templates added as BASE.md, SITE_BASE.md, SITE.md and PDF.md;
added docker scripts for automatic publication of the site and PDF, create-site.sh and create-pdf.sh;
added ./docs/Gemfile and docs/_config.yml to support GitHub Pages publication;
removed unnecessary intro to the markdown source specification/CITS_Geospatial_v3.md;
added publication metadata file specification/metadata.md; and
removed old CNAME which isn't needed for GitHub Workflow publication.

DILCISBoard / CITS-Geospatial