Update the SCM User Guide for v7 release

[scrasmussen]

The TechGuide needs to be updated for the upcoming release.

Items to change:

• [x] v6.0.0 references need to be updated to v7.0.0
• [x] "Section 3.1 Version Notes" needs to be updated with information on the new major and minor changes
• Remove/update Cheyenne references to Derecho
  • [X] Build instructions have been updated
  • [x] Still references in 7.5.3, 7.5.6, 4.2
• Sections that may need updating:
  • [x] New cases in "Section 7.2: Included Cases"?
• Others?
• Switch from LaTeX to ReadTheDocs/sphinx?
  • [X] #418
• [ ] Remove/update references to support forum to GitHub Discussions instead