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>