Merging upstream version 5.3.0+dfsg.

Signed-off-by: Daniel Baumann <daniel@debian.org>

site/content/docs/5.3/getting-started/best-practices.md

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+---
+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.
+
+{{< callout info >}}
+**Heads up!** This copy is a work in progress.
+{{< /callout >}}
+
+### General outline
+
+- Working with CSS
+- Working with Sass files
+- Building new CSS components
+- Working with flexbox
+- Ask in [our GitHub Discussions](https://github.com/twbs/bootstrap/discussions)