Get new help.gnome.org running with Pintail

Pintail is the documentation site builder made for Mallard. We're working on moving help.gnome.org to Pintail.

Resources:

• Pintail repo on GitHub
• help.gnome.org repo with config for our deployment
• test instance built from above repo.

Tasks:

• Get a nice design for the front page.
• Get search working. This likely requires making a pintail-solr plugin to replace the problematic pintail-elasticsearch plugin.
• Ensure translations are working.
• Language selector in the sidebar.
• Get translations of the front page.
• Add unstable builds, with custom styling to show what they are.
• Tell search engines not to index unstable builds.
• Figure out rebuild schedule, or if triggered by CI.
• Deploy!