v1.22.1: avoid panic when Pebble restarts after stop-checks

This is a bugfix release that prevents a panic when the Pebble daemon restarts after a stop-checks operation (#625).

It also includes a minor change to the client package to always add Content-Type: application/json to requests (#619), and an update to the docs starter pack and build scripts (#615).

Full Changelog: v1.22.0...v1.22.1