Publish user docs to GitLab pages
This MR does this:
- Creates translated Mallard docs as part of the build process.
- Renders those translated Mallard docs to HTML.
- Creates a minimal landing page.
- Publishes the HTML to https://gnome.pages.gitlab.gnome.org/orca
Each translation will be available like this:
- https://gnome.pages.gitlab.gnome.org/orca/help (English, default)
- https://gnome.pages.gitlab.gnome.org/orca/es (Spanish)
- https://gnome.pages.gitlab.gnome.org/orca/fr (French)
- etc.