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/