Example for Open Geodata

[fretchen]

In this PR, I added documentation on the usage of open geodata. The result can already be visualized in this test environment.

• It visualizes how different datasets can be integrated.
• It is based on a relevant context.
• It uses creative commons compatible datasets only.
• The code is kept as simple as possible.

[Maja4Dev]

LGTM. I suggest to merge.