common: deprecate dataValue

[josephjclark]

And maybe some similar functions (sourceValue? asData?) - although we have to be careful that they are not used internally.

I don't think they're particularly useful in modern job writing and should be de-listed, removing clutter from the docs.

Another approach to these would be to mark them private, so they're just hidden from docs.

As part of this work, we need to go through the docs and remove example code that uses dataValue

[josephjclark]

Speaking with Taylor this morning we are almost ready to remove dataValue and focus on lazy state as the official idiomatic way to read state.

One possible problem with dataValue is that the docs say it's an Operation (well, a function that returns an operation). But the signature is not (path) => (state) => State, it's signature is (path) => (state) => any, where any is some value, usually a primitive.

The point is that dataValue returns a function that takes state and returns a value. It does not take state and return state.

It's a small pedantic detail but it becomes quite important when splitting the docs into Operations and Functions, like in #613