partials: Add reusable macros for all D&I team docs
This commit adds a `_partials/attributes.adoc` file. This is a special
file that includes macros that can be reused across all Antora docs
pages. To reuse any of these macros, you wrap them around curly
brackets.
For example, to get https://communityblog.fedoraproject.org to appear on
a page, write `{{COMMBLOG}}` to embed the macro.
Signed-off-by: Justin W. Flory <git@jwf.io>