Replace mkdocs by Antora

Refs https://redmine.teklia.com/issues/7248

This MR convert the mkdoc content to Asciidoc and replace mkdocs by Antora.

The main change is that I'm not able to generate the source code reference

Otherwise all redacted content is ported over to Asciidoc, search is working, and I added a "Community" button to link towards forum & repo.

You can view the current build at https://antora-teklia-atr-pylaia.surge.sh/