Update README, wiki; delete build code

[AlexanderRichert-NOAA]

The documentation in this repo's README as far as supported compilers, etc. should be updated. Let's at least flag the "code" here (CMake config etc.) as deprecated/use at own risk.

Remove:

• Most of the instructions from the README, apart from Spack discussion

Add/update:

• Use-at-own-risk notice (README; CMakeLists.txt)
• Supported compilers (README)
• CI discussion (wiki)
• Link to spack-stack repo & docs (README)
• Code/repo policies & best practices (wiki)
• List of links to NCEPLIBS-using codes
• Delete old build scripts; this repo should pretty much just be README & usage_tracking (plus issues, wiki, etc.)