New user guide (experiment flow)

Explanation of changes

Closes #96 (closed) , documents #66 (closed)

Documentation on how the different concepts tie together. WIP. I've rewritten the user guide (see RTD preview) and streamlined the overall experience. It is less about the data structures and more about the concepts.

This MR is still WIP, but I would very much like some feedback on this already.

• Introduction
• Creating a schedule (example could be better, but out of scope of this MR)
• Compilation
• Execution -> Instruments
• Execution -> experiment flow (figure ready, text to be written).

I will also delete most of the old user guide (still present in this MR).

@kel85uk, @dcrielaard, @caenrigen, @adamorangeqs, @jvoven please let me know what you think.

Motivation of changes

Overdue design work that is blocking a lot of progress for the current release.

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• Merge request contains a clear description of the proposed changes and the issue it addresses.
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• New code is documented and docstrings use numpydoc format.
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