Pico 2.1 mostly targets the mostly completed rewrite of Pico CMS for Nextcloud, which is going to be a officially supported by Pico. This is a minor release. This release includes new features, but doesn't break BC with older releases of Pico when using Pico's official PicoDeprecated plugin.
Please use this PR for code reviews only, for a list of major changes and for comments please refer to https://github.com/picocms/Pico/pull/515