One branch with source and result
The site is a Jekyll site in the asf-site branch of apache/wicket-site. The branch holds two things: the source, Markdown pages with their layouts and data, and in content/ the HTML that Jekyll builds from it. Whoever changes the source builds the site and commits both.
The ASF’s web servers publish content/ as wicket.apache.org. The .asf.yaml file at the root of the branch asks for this with publish: whoami: asf-site, and the ASF’s publishing service picks up every commit to the branch within a minute. No build server is involved: what is committed in content/ is what visitors get.
That is why the site is built on your machine and not by the ASF. Many ASF sites use a build service that only knows Pelican, or a GitHub Actions workflow that builds and pushes the result. Wicket’s site uses its own Jekyll plugins, for the documentation’s indexes and the news archives, so it is built locally with the gems in the Gemfile.
Build and serve are not the same
jekyll build builds the site for wicket.apache.org: the url in _config.yml is https://wicket.apache.org, and feeds and links that need a full address use it. jekyll serve builds the site for your machine and replaces that address with localhost. Anything you commit to content/ has to come from jekyll build, or the Atom feed would send readers to localhost. For local previews, jekyll serve -d /tmp/site keeps that copy out of content/.
Data instead of pages
With a major release every quarter and patch releases in between, the site changes often, and the people who change it are volunteers releasing Wicket, not web developers. So a release is data: the version numbers in _config.yml, the release lines in _data/releases.yml, the highlights in _data/highlights.yml, and a post. The version plate in the menu, the release lines page, the line pages, the panels on the documentation and the quick start are generated from that data. A release updates all of them without anyone editing a page, and none of them can fall behind another.
The documentation follows the same idea with versions. A page states the oldest line it is correct for in since, and paragraphs that differ between lines carry a note. One page serves every line in service, a reader filters by the version they use, and a line leaving service needs no edit at all.
Redirects keep old links working
Release announcements, mailing list archives and answers elsewhere link to the site for years. The web server applies .htaccess, which Jekyll copies into content/, so a moved page keeps its old address as a redirect. See Redirect an old URL.