As part of the initiative to sunset the use of Autotools, starting with the GNOME 51.alpha release the gnome-user-docs project, which contains the user and sysadmin documentation, has switched to Meson.
A notable change in the build, compared to the old Autotools one, is that tests are now built by default, which adds a build time dependency to xmllint and yelp-check. You can disable tests by adding -Dtests=false to the meson setup options.