Currently buried in the commit message for 13d2869.

• checkout the archive branch
• ensure there's no remaining generated content under docs & addons
• change available versions here:
  https://github.com/openhab/website/blob/743f0b867e4d26a5a6a3116a12f6344295ef25d6/.vuepress/components/DocPreviousVersions.vue#L72
  (note to self: use frontmatter/root level config in the future?)
• set OH_DOCS_VERSION=2.4
• ruby prepare-docs.rb
• vuepress build
• checkout master again & create a PR branch
• copy the site in the vuepress folder under .vuepress/public/v2.4
• push, open the PR and merge