cmake/README: Some cleanup

[VelocityRa]

Description

Deletes some duplicated stuff and moves docs for a bunch of CMake vars and some build instructions one directory higher.

Motivation and Context

They're more visible/accessible this way, they shouldn't have been cmake/addons in the first place.

Types of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Clean up (non-breaking change which removes non-working, unmaintained functionality)
• [ ] Improvement (non-breaking change which improves existing functionality)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that will cause existing functionality to change)
• [x] Cosmetic change (non-breaking change that doesn't touch code)
• [ ] None of the above (please explain below)

Checklist:

• [x] My code follows the Code Guidelines of this project
• [ ] My change requires a change to the documentation, either Doxygen or wiki
• [ ] I have updated the documentation accordingly
• [x] I have read the Contributing document
• [ ] I have added tests to cover my change
• [ ] All new and existing tests passed

[VelocityRa]

Wrong remote, sorry!