Editorial: Define type definitions

[leebyron]

This broad change uses definition phrases to define each type, as well as expands the set of sub-sections at the top of the "Types" section which define kinds of types. In doing so applies the definitions arrived at in #845 for "wrapped types".

Makes editorial improvements throughout, removing duplicate language or attempting to improve correctness or legibility.

Closes #826 Closes #845

[benjie]

@leebyron I think some unrelated changes have made their way into this PR, e.g. signed-agreements/README.md

[benjie]

I have now completed the review but have no further feedback :+1:

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