Rearrange all the docs to allow for a docs/major.minor/ setup

docs/4.0/extend/approach.md

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+---
+layout: docs
+title: Approach
+---
+

docs/4.0/extend/icons.md

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+---
+layout: docs
+title: Icons
+description: Guidance and suggestions for using external icon libraries with Bootstrap.
+group: extend
+---
+
+Bootstrap doesn't include an icon library by default, but we have a handful of recommendations for you to choose from. While most icon sets include multiple file formats, we prefer SVG implementations for their improved accessibility and vector support.
+
+## Preferred
+We've tested and used these icon sets ourselves.
+
+- [Iconic](https://useiconic.com/open/)
+- [Octicons](https://octicons.github.com/)
+- [Entypo](http://www.entypo.com/)
+
+## More options
+While we haven't tried these out, they do look promising and provide multiple formats—including SVG.
+
+- [Google Material icons](https://material.io/icons/)
+- [Ionicons](http://ionicons.com/)
+- [Feather](http://colebemis.com/feather/)
+- [Dripicons](http://demo.amitjakhu.com/dripicons/)
+- [Ikons](http://ikons.piotrkwiatkowski.co.uk/)
+- [Glyph](http://glyph.smarticons.co/)

docs/4.0/extend/index.md

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+---
+layout: docs
+title: Extend
+---
+
+todo: this entire page