Add materials for geo aggregation

[simonprickett]

Add materials for geo aggregations including the new polygon functionality when that is available. Also point to any external developer relations information that we produce as extra learning material.

[simonprickett]

We'll need a small extension to the dataset to include "libraries" for this, will need its own index, amendments to the data loader and new materials covering radius and polygon searches. Add this to the aggregate search section 3.4.

[simonprickett]

Actually these are in ft.search now which is better.