Port docs to gi-docgen
This MR ports the documentation generator from gtk-doc to gi-docgen. I've also "ported" the content pages from gtk-doc format to markdown. I did also add a basic CI configuration that publishes the documentation to Gitlab pages. You can see a preview here: https://vixalien.pages.gitlab.gnome.org/grilo. Before merging this, it'd be smart to ensure the pages
job is only run on the main
branch.