Add a CI pipeline for building, validating, and publishing

We run the build using Meson, split into three stages:

 - validation
 - build
 - deployment

The validation phase runs the build with -Dvalidation=true; the build
phase runs the build with just the HTML generation and without
translations, for the moment; the deployment phase takes the build
artifacts and publishes them on the GitLab pages space for the project.