Split the getting started docs

[tovrstra]

This fixes one of the tasks in #191 and is also useful for #254 and #210.

The getting started guide is split in smaller bite-size parts. This also creates some more room for extending the getting started with a few more examples as mentioned in some of the cited issues.

The contents of the guide has not really changed, except maybe for the text in gettingstarted/index.rst

I will YOLO-merge this on Friday, June 28, unless reviewed earlier.

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[tovrstra]

Thanks for checking. Merging...