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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[deepsource-io]

Here's the code health analysis summary for commits b5bccd9..ce38390. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Shell	✅ Success		View Check ↗
Python	✅ Success		View Check ↗

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[sourcery-ai]

Hey @tovrstra - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟡 Documentation: 1 issue found

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[PaulWAyers]

Thanks Toon. I especially appreciate the way the units is explained clearly, with the rationale of using old-timey notation.

[tovrstra]

Thanks for checking. Merging...