Design Documentation Update

[Eunovo]

This PR contains an attempt to improve the design documentation to help new contributors to the repo. It adds more details about BuildField, ReadField and PassField. It explains how Callbacks, ThreadMaps and Async processing work.

[DrahtBot]

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Reviews

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Type	Reviewers
Concept ACK	Sjors
Stale ACK	ryanofsky

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Possible typos and grammar issues:

• resuse -> reuse [spelling error]
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[ryanofsky]

@theuni you may be interested in this, it describes things like how callbacks can be implemented

[enirox001]

This is a helpful update. As someone getting familiar with this, the new details really helped clarify the design.

[Sjors]

Concept ACK on expanding documentation, and thanks for adding these graphics.

Reviewer hint, switch to the rich diff:

@ryanofsky is probably best qualified to check correctness.

[ryanofsky]

Code review ACK 459a730. These updates are very good, and I think information about callbacks and context parameters should be especially helpful. I left a number of suggestions below, which can be addressed here or in a followup. You can let me know which you prefer.

[Eunovo]

Thanks for the review @ryanofsky . I made the suggested changes.