Introduce a collection for maps

Jekyll supports more then just one collection of posts. See also the [documentation about collections](https://jekyllrb.com/docs/collections/).

We can therefore create a collection of maps. These would not just be any maps, but maps that we consider remarkable and we want the community to be able to find them. To make this more concrete:

- Add a new collection called 'maps'.
- Add a new layout specifically for an item of the maps collection.

Each post in the maps collection should have the following fields in the front matter:

- The name of the map
- The version of the map
- The description of the map
- The reclaim values on the map
- A thumbnail of the map

The post itself can contain various other media elements such as text, images and embedded video's.