Define field and future improvements

[Dhghomon]

Closes #183, addresses one item in #559

This PR mentions that futures can be included in schema definitions, but also rewrites the DEFINE FIELD page a bit in three other ways:

1. General improvements like removing old syntax
2. Rearranging sections for a bit more unified narrative
3. Mentions that VALUE is recalculated every time an update happens (users have mentioned wanting to see this on Discord before, though no official issue has been created) and then uses that as the launching point to mention how futures inside a schema work.

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