Document AsciiDoc attributes set by Antora

Example: page-component-version

It might help to set standard AsciiDoc attributes such as revnumber accordingly.

I learned about these attribute from here: https://gitlab.com/antora/antora/blob/master/packages/asciidoc-loader/lib/load-asciidoc.js

Should also make the connection to: https://docs.antora.org/antora-ui-default/templates/#template-variables