stub out a best practices page

docs/getting-started/best-practices.md

+---
+layout: page
+title: Best practices
+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.
+{% endcallout %}
+
+### General outline
+- Working with CSS
+- Working with Sass files
+- Ask in Slack