New plugin: pelican-po4a

[Natureshadow]

I have written a plugin that integrates pelican and po4a. It has a few rough edges, but overall, it does what it is supposed to do.

The plugin can be found here:

https://edugit.org/Teckids/team-pr/pelican-po4a

As explained in the repository's CONTRIBUTING.md, I would like to keep the development on EduGit (because the plugin has contributors who are locked out by GitHub's terms of use). I would, however, like to mirror it to the pelican-plugins GitHub organisation, if that is desired.