ci: Add a pipeline for building the website

See merge request !3

We're getting 404s when cloning all repositories; instead of just dying,
print out where we failed.

The configger script is bailing out when asking GitLab about the Evince
project branches, and failing the pipeline because it tries to get the
"master" branch.

# Conflicts:
#   pintail.cfg

Also added a couple icons, and shotwell to the built apps

We only want to change some details and options between the C locale and
the full rebuild jobs, so there's no point in copy-pasting the same
script rules between them, when we can use `extends`.

Use the `--no-translation` CLI option instead of changing the
configuration file.

- Drop translations
- Drop the gnome-help 3.32 branch
- Drop the sysadmin guide

Build only the default language on all refs, and the full translated
site on the main development branch.

Between cloning and localisation, the default of 1 hour isn't enough.

Avoids pintail trying to add the tests from the dependencies to the
build.

To allow the configger.py script to run.

Download all the dependencies and then run pintail.

The output is then stored as an artefact.

We had APPS.xml being read by the config script, and that was better
than writing boilerplate git repos over and over. But we sitll had to
manually track branches. No more. Now it's an even more minimal YAML
file, and it's read by a Python script that can ask GitLab about
branches. If you use standard GNOME branch names, it's automatic.

Current plan is for stable docs to not include a version slug in
the URL, and for unstable/master docs to be under unstable/docname.
Not currently planning on old stable versions, but if we add them,
they'll be under version/docname.

Fix repo URLs in pintail.cfg & Update README with more dependencies

See merge request shaunm/help.gnome.org!2