f7b4265 asciidoc-fedora: Update partials docs for Antora 2.0

Authored and Committed by jflory7 3 years ago
    asciidoc-fedora: Update partials docs for Antora 2.0
    
    This commit refactors the documentation on how to use reusable
    attributes. Recently I needed to create reusable attributes in a docs
    project with multiple modules (a.k.a. this repo). So, while digging, I
    discovered that the way I learned to include partials is deprecated
    since Antora 1.1. The current stable version is 2.3.4 and an early alpha
    is released for v3.0.0.
    
    So, this is a slight modernization to make sure we are giving people the
    right advice to set up their partials in a way that will continue to
    work in future versions of Antora. Hurrah!
    
    References:
    
    * https://docs.antora.org/antora/2.3/partials-directory/
    * https://docs.antora.org/antora/2.3/page/partials-and-content-snippets/
    * https://docs.antora.org/antora/2.3/page/include-a-partial/
    * https://docs.antora.org/antora/2.2/asciidoc/page-to-page-xref/
    
    Signed-off-by: Justin W. Flory (he/him) <git@jwf.io>