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/getting-started/best-practices.md

@@ -0,0 +1,21 @@
+---
+layout: docs
+title: Best practices
+description: Learn about some of the best practices we've gathered from years of working on and using Bootstrap.
+group: getting-started
+---
+
+We've designed and developed Bootstrap to work in a number of environments. Here are some of the best practices we've gathered from years of working on and using it ourselves.
+
+{% capture callout %}
+**Heads up!** This copy is a work in progress.
+{% endcapture %}
+{% include callout.html content=callout type="info" %}
+
+### General outline
+
+- Working with CSS
+- Working with Sass files
+- Building new CSS components
+- Working with flexbox
+- Ask in [Slack](https://bootstrap-slack.herokuapp.com/)