Update XFCE document to include procedure for 9.0

[sspencerwire]

• Make minor changes to the original 8.6 document by formatting code blocks consistently and prefixing items so that they are not confused in the right-hand TOC with those associated with 9.0
• Softened the language in the conclusion section and made it inclusive with the 9.0 changes
• moved 8.6 section to tabbed content
• Added 9.0 section as tabbed content with similar language as used in the MATE 9.0 changes
• NOTE: the EULA screen did not come up when installing XFCE, so I've left that screen shot and subsequent text out.
• Added to the meta tags at the beginning of the document to indicate tested versions Author and current contributors.

Author checklist (Completed by original Author)

• Contribution a good fit for the Rocky project? Title and Author MetaTags inserted ?
• Is this a non-English contribution?
• If applicable, steps and instructions have been tested to work on a real system
• Did you perform an initial self-review to fix basic typos and grammatical correctness

Rocky Documentation checklist (Completed by Rocky team)

• 1st Pass (Check that document is good fit for project and author checklist completed)
• 2nd Pass (Technical Review - check for technical correctness)
• 3rd Pass (Basic Editorial Review)
• 4th Pass (Detailed Editorial Review and Peer Review)
• Final pass/approval (Final Review)

[sspencerwire]

@SergeCroise Feel free to look this one over as well. I've tried to apply your previous suggestions to this document, so hopefully, there are no major changes needed. The 8.6 document could need changes. I was not the author of the 8.6 document.
@EzequielBruni If you find anything that needs changing, it will likely need to be changed in the MATE document already merged.
I appreciate both of your efforts!