📝 index.adoc: Introduce partials, refresh for 2023

This commit is a simple refresh to the default landing page for the Fedora Mindshare Committee documentation. The biggest change to actual content was rewriting the section about how to contact us and about our ticket-based workflow. These now emphasize Discourse, Matrix, and GitLab as the primary methods to work with the Mindshare Committee.

Additionally, this commit also introduces the use of attributes.adoc for shortcodes for frequently-used data and links. For example, links to the various communication platforms are entered as attributes that can be referenced from one common source of data whenever it is invoked. If there are ever changes to the URLs, this makes it easier to fix broken links in documentation from one place.

EDIT: A pre-rendered preview is available for one week (expires 18 April 2023) from this URL.