Enable Continuous Delivery of the preview website from the "master" branch + Bulk cleanup

[oleg-nenashev]

Just a quality of life improvement. Full continuous deployment automation is coming later, but at least we will have a preview site where we could verify the status. I do not want to put too much effort, because ultimately I'd like Jekyll to be replaced by Docusaurus or, as @tomakehurst prefers, Hugo. FTR #24 for the migration.

Preview: https://wiremock.github.io/wiremock.org-sources/

Chore

• Add CD to GitHub Pages: https://wiremock.github.io/wiremock.org-sources/
• Dependency updates so that it builds on latest Ruby 2.x, Jekyll and Bundler
• Switch from base_path to modern abolute_url filter to fix non-root-URL deployments
• Fix main.css documentation generation which was (most likely) broken in the recent versions
• Fix Gemlock that breaks on Setup Ruby (currently a hackaround instead of proper setup_ruby GitHub Action use)

Content

• Fix OpenGraph and other metadata which was broken in the recent site versions - Website opengraph is broken #26
• Fix many broken absolute and subsection links (but not all of them)

[oleg-nenashev]