Move notices, disclaimers, descriptions, introductions, etc. to <front-matter> and or <notice> elements

[mpreisler]

front-matter, rear-matter, notice elements and possibly Benchmark description all exist in XCCDF just for this purpose. I think we should use them instead of abusing Group descriptions for this purpose.

The main reason for this ticket is the fact that these filler groups look weird and confusing in SCAP Workbench:

Users usually ask this:

• What are these for?
• Should I select them?
• What are the consequences of unselecting them?
• Are these different between profiles?
• If they have no effect on evaluation, why am I even allowed to select or unselect them?

I am proposing to replace the Introduction group and put its contents in front-matter and a set of notices.

[shawndwells]

makes sense to me. IIRC, there was some reason NSA needed the text in the group descriptions vs front-matter in order to produce the NSA SNAC Guides.... @jeffblank? @davesmith2?

[yuumasato]

I was recently confused by these groups, it makes sense to move them out of groups.
The SSG build system should be updated to put this information in the front-matter, rear-matter, etc. and openscap needs to be checked if it supports these elements.