switch to Pelican-plugins organization for plugin sources

[maphew]

I've had some troubles getting up and runing using the blog-o-matic template. One of the issues revolved around the Liquid-tags plugin, which led me to learning that the "Pelican community started to move away from this repository into separate pelican-plugins organization with Pelican 4.5, released in 2020" - from pelican-plugins/liquid-tags#38

Its marquee feature is support for “namespace” plugins, which means any plugin listed under the new Pelican Plugins organization can be installed via Pip, after which Pelican should automatically detect and enable the new plugin.