Update CONTRIBUTING.md docs with up to date data:
- Discord invite to new sif server
- Update URL-s to new vmfunc/sif repository
- Update guidelines on contributing framework detection patterns
- fix version detection to validate reasonable version numbers (major < 100)
- remove overly permissive patterns that caused false positives
- add comprehensive framework contribution documentation to CONTRIBUTING.md
- document signature patterns, version detection, and CVE data format
- add configuration documentation for flags and env vars
- outline future enhancements for community contributions
