Add section explaining AsciiDoc attribute files

[inoxx03]

The content for this section should briefly explain:

• what an attributes.adoc file is and why is it useful when creating books
• what necessary elements that an attributes file must contain (for instance, doctype must be specified)
• A breakdown of the structure of a simple attributes.adoc. (an embedded adoc source with callouts on important lines would be the fomatting I'd prefer, but I'm open to suggestions here:))
• Examples of more advanced attribute file usage, such as defining replaceable variables for book, chapter,component or package names, URLs, etc ,...
• Also, it might be useful for the section to demonstrate how replaceable variables can be further used to pre-defined create link URLs

Please feel free to add content suggestions for useful information that the chapter should include, if anything comes to your mind:)

[ncbaratta]

Looks like we might have this already in the guide: https://redhat-documentation.github.io/community-collaboration-guide/#ccg-asciidoc-attributes but it could use some beefing up probably.

[inoxx03]

@nengard Right, my bad:) I completely skipped over that one and missed it when checking the guide.I'll go over the existing content to see where and how it can be improved:)

[ncbaratta]

No worries! I did too! I'm the one who said we didn't have it in the guide yet :)

[jenmalloy]

+1 to @inoxx03 suggestions, in particular how to "build out" link URLs with replaceables/variables. It's a great way to avoid hardcoding and really powerful for that reason.

Also, how to handle the product attributes file would be useful, since that's where things can get tricky if you are mirroring the upstream repo downstream. Since I'm working through this right now, I can write up my findings after I get it all figured out.

[inoxx03]

I've made some additions to the AsciiDoc chapter. The changes are introduced by PR #94.