Add NuGet 6.8 release notes to TOC (#3192)

Author: zivkan

docs/TOC.md

@@ -339,6 +339,7 @@
 ### [Known Issues](release-notes/known-issues.md)
 
 ### NuGet 6.x
+#### [NuGet 6.8](release-notes/NuGet-6.8.md)
 #### [NuGet 6.7](release-notes/NuGet-6.7.md)
 #### [NuGet 6.6](release-notes/NuGet-6.6.md)
 #### [NuGet 6.5](release-notes/NuGet-6.5.md)