major update to README.md plus two new child docs

[Grammaresque]

links to the child docs from the main readme file will be broken until the child docs are approved, moved to their appropriate location within the tree, and given a url. thank you.

Author checklist (Completed by original Author)

• Good fit for the Rocky Linux project? Title and Author Metatags inserted ?
• If applicable, steps and instructions have been tested to work
• Initial self-review to fix basic typos and grammar completed

Rocky Documentation checklist (Completed by Rocky team)

• 1st Pass (Document is good fit for project and author checklist completed)
• 2nd Pass (Technical Review - check for technical correctness)
• 3rd Pass (Detailed Editorial Review and Peer Review)
• Final approval (Final Review)

[wsoyinka]

Thank you @Grammaresque :-)

[github-actions]

Test results for c0281fd:

Number of broken URLs: 3

URL,RESULT,FILENAME
 https://dl.rockylinux.org/pub/rocky/9.0/live/x86_64/,failed,guides/desktop/xfce_installation.md
 https://dl.rockylinux.org/pub/rocky/9.0/live/x86_64/,failed,guides/desktop/kde_installation.md
 https://dl.rockylinux.org/pub/rocky/9.0/live/x86_64/,failed,guides/desktop/mate_installation.md