Fix KSP-AVC links and other small issues in spec

[HebaruSan]

Noticed a few more minor issues after #3519:

• The resources.homepage in the example CKAN file broke due to forum updates and is now updated to AJE's current thread
• "version ordering" should use sentence case capitalization because it's not a metadata property, unlike the other lowercase headings
• The link to Jenkins was using HTTP and is now updated to HTTPS
• The link to the KSP-AVC spec broke when Cybutek removed it from ksp.cybutek.net, so it now points to the README.md file in the current maintainer's repo, which should offer the same content (and the URL is now shared between the two places where it's used via Markdown magic, for easier maintenance)
• The documentation for $vref now mentions that it sets resources.remote-avc (see Store remote version file URL in metadata resources #3259)