Gem 'bigdecimal' must be installed

I followed the documentation about installing and configuring the PDF exporter plugin and got the following error:

$ antora -r @antora/pdf-extension antora-playbook.yml --stacktrace
[...]
cannot load such file -- bigdecimal (LoadError)
[...]

I am not so much into Ruby but from what I understood is, that 'bigdecimal' was removed from the Ruby core and needs to be installed by the user now. As it is a native extension, it must be compiled.

This should mentioned in the documentation.