Use new grid syntax (#545)

Author: jarrodmillman

doc/content/_index.md

@@ -4,28 +4,28 @@ title:
 
 {{< grid columns="1 3 3 3" >}}
 
-{{< card >}}
+[[item]]
+type = 'card'
 title = 'Documented'
 body = '''
 The theme is documented at https://theme.scientific-python.org.
 '''
-{{< /card >}}
 
-{{< card >}}
+[[item]]
+type = 'card'
 title = 'Reusable'
 body = '''
 The theme is being used by
 [scientific-python.org](https://github.com/scientific-python/scientific-python.org),
 [numpy.org](https://github.com/numpy/numpy.org), and
 [scipy.org](https://github.com/scipy/scipy.org).
 '''
-{{< /card >}}
 
-{{< card >}}
+[[item]]
+type = 'card'
 title = 'Community Maintained'
 body = '''
 The theme is maintained by the [Theme Team](/about/).
 '''
-{{< /card >}}
 
 {{< /grid >}}