Adopt Diátaxis Framework for documentation

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Diátaxis Framework provides useful guidelines on how to develop, present and maintain technical documentation.

To a large extent I think we are already following many of these principles. This issue is to go through the Diátaxis Framework and ensure that we do.

For intance, Diátaxis emphasises to write differently for:

• Tutorials
• How-to guides
• Reference
• Explanation

We are kind of doing all these things in RO-Crate website and specification, but perhaps we are not clearly enough separating them.