Prep for v4.2.0 release

- Move 4.1 docs to 4.2
- Update versions everywhere to 4.1.3 with release script
- Manually bump the shorthand version in package.json
- Add 4.2 to the versions docs page
- Update some redirects
- Fix tests asset URLs
- Bump Nuget and more

site/docs/4.2/extend/index.md

@@ -0,0 +1,7 @@
+---
+layout: docs
+title: Extend
+redirect_to: /docs/4.1/extend/approach/
+---
+
+todo: this entire page